diff --git a/modules/mesh-delivery-indexer/cmd/mesh-delivery-indexer-tools/main.go b/modules/mesh-delivery-indexer/cmd/mesh-delivery-indexer-tools/main.go
index 464a1921..2fc39c4d 100644
--- a/modules/mesh-delivery-indexer/cmd/mesh-delivery-indexer-tools/main.go
+++ b/modules/mesh-delivery-indexer/cmd/mesh-delivery-indexer-tools/main.go
@@ -129,9 +129,10 @@ func Tools(ask asker, now func() time.Time, read stdio.KeptState, counts func()
 			}},
 		{Name: index.SeatDefinitionsAgainTool,
 			Description: "Read a contracts module's seat definitions again now (design 26): its trunk's, at the commit the indexer " +
-				"holds the module at, or with pull and head a pull request's head, and keep them in the seat-definitions state as " +
-				"a merge or a pull request's push would (seat-definition-changed said where a digest moved; the trunk's " +
-				"seat-definition-unparsed finding written or cleared). For a read the forge did not answer. A hand act, " +
+				"holds the module at, or with pull and head a pull request's head (read with the head's own manifest, so its " +
+				"generated-from as the head has it), and keep them in the seat-definitions state as a merge or a pull request's " +
+				"push would (seat-definition-changed said where a digest moved; the trunk's seat-definition-unparsed and " +
+				"seat-defined-twice findings written or cleared). For a read the forge did not answer. A hand act, " +
 				"recorded through mesh-hand-acts before it is asked. indexer_seat_definition reads the result.",
 			Input: map[string]any{"type": "object", "required": []string{"module", "why"}, "properties": map[string]any{
 				"module": map[string]any{"type": "string", "description": "the contracts module, such as mesh-contracts"},
diff --git a/modules/mesh-delivery-indexer/internal/index/control_records_test.go b/modules/mesh-delivery-indexer/internal/index/control_records_test.go
index 06357b29..d0f73bf7 100644
--- a/modules/mesh-delivery-indexer/internal/index/control_records_test.go
+++ b/modules/mesh-delivery-indexer/internal/index/control_records_test.go
@@ -182,6 +182,42 @@ func TestTheControlRecords(t *testing.T) {
 		if n := len(said); n != 4 || said[2] != (SeatDefinitionChanged{Seat: "gone", Source: SourceTrunk, Repository: repo}) || said[3].Digest == "" {
 			t.Errorf("said %+v, want gone's removal and m3's git", said)
 		}
+		// Ruling (lead, on A2i's finding 4): another repository's trunk defining git is refused, the trunk's entry stands,
+		// nothing is said, and its finding names both repositories; its next merge without git clears the finding.
+		const other = "novox/other-contracts"
+		files["o1"] = map[string]string{"seats/index.json": `{"format":1,"seats":["git"]}`, "seats/git/seat.json": strings.Replace(git, "ADR 0111", "ADR 0300", 1)}
+		files["o2"] = map[string]string{"seats/index.json": `{"format":1,"seats":[]}`}
+		read := ix.ReadFile
+		ix.ReadFile = func(ctx context.Context, r, sha, p string) ([]byte, error) {
+			if r == other {
+				r = repo
+			}
+			return read(ctx, r, sha, p)
+		}
+		otherManifest := strings.Replace(manifest, `"mesh-contracts"`, `"other-contracts"`, 1)
+		mergedOther := func(merge string) {
+			t.Helper()
+			if err := ix.ApplySeatDefinitions(ctx, delivery.Forge{Kind: delivery.ForgeMerged, Repository: other, Number: 2, Merge: merge,
+				Manifests: map[string]string{delivery.RootPath: otherManifest}}); err != nil {
+				t.Fatal(err)
+			}
+		}
+		twice := health.FindingKey(health.ScopeModule, findingID("seat-defined-twice", "other-contracts"), SeatDefinedTwiceKind)
+		before := len(said)
+		mergedOther("o1")
+		if g, _ := entry("git.trunk"); g.Repository != repo || g.Commit != m3 || len(said) != before {
+			t.Errorf("a second repository's git replaced the trunk's entry or was said: %+v, said %+v", g, said[before:])
+		}
+		if v, ok := findings.vals[twice]; !ok || !strings.Contains(string(v), other) || !strings.Contains(string(v), repo) {
+			t.Errorf("finding %s = %s, want it naming both repositories", twice, v)
+		}
+		mergedOther("o2")
+		if _, ok := findings.vals[twice]; ok {
+			t.Error("the seat-defined-twice finding stands once the second repository defines git no more")
+		}
+		if g, ok := entry("git.trunk"); !ok || g.Repository != repo {
+			t.Errorf("the second repository's merge without git removed the first's entry: %+v", g)
+		}
 		// A pull request's head: its own entry, the trunk's answered unless it is named.
 		if err := ix.ApplySeatDefinitions(ctx, delivery.Forge{Kind: delivery.ForgePull, Action: delivery.PullUpdated, Repository: repo, Number: 9, Head: head,
 			Dirs: []string{delivery.RootPath}, Manifests: map[string]string{delivery.RootPath: manifest}}); err != nil {
@@ -225,6 +261,25 @@ func TestTheControlRecords(t *testing.T) {
 		if len(findings.vals) != 0 {
 			t.Errorf("findings %v stand after a clean build end", findings.vals)
 		}
+		// Ruling (lead, on A2i's finding 3): indexer_seat_definitions_again for a pull request reads the head's own manifest's
+		// generated-from: a trunk that names none, a head that names it, is read as the contracts module the head is.
+		ix.Modules, ix.Manifests = newMem(), newMem()
+		plain := `{"module":"mesh-contracts","version":"1"}`
+		kept, _ := delivery.EncodeManifest(delivery.Manifest{Module: "mesh-contracts", JSON: plain})
+		mkey, _ := delivery.ManifestsKey(delivery.ManifestDigest([]byte(plain)))
+		ix.Manifests.(*memStore).vals[mkey] = kept
+		mod, _ := delivery.EncodeModule(delivery.Module{FP2: "src2:" + strings.Repeat("a", 64), Manifest: delivery.ManifestDigest([]byte(plain)), Repository: repo, Commit: m3, Path: delivery.RootPath})
+		ix.Modules.(*memStore).vals["mesh-contracts"] = mod
+		files["h2"] = map[string]string{"module.json": manifest, "seats/index.json": `{"format":1,"seats":["git"]}`, "seats/git/seat.json": strings.Replace(git, "ADR 0111", "ADR 0400", 1)}
+		if r := ix.SeatDefinitionsAgain(ctx, "mesh-contracts", 9, "h2"); !r.Stored {
+			t.Errorf("again for the pull request's head = %+v, want it read with the head's generated-from", r)
+		}
+		if e, ok := entry(pk); !ok || e.Commit != "h2" {
+			t.Errorf("the pull request's entry after again = %+v (%v)", e, ok)
+		}
+		if r := ix.SeatDefinitionsAgain(ctx, "mesh-contracts", 0, ""); r.Stored || !strings.Contains(r.Answer, "trunk's manifest") {
+			t.Errorf("again for the trunk, whose manifest names no generated-from = %+v", r)
+		}
 		// The pull request closes: its entries and its out-of-step files go, said.
 		if err := ix.ApplySeatDefinitions(ctx, delivery.Forge{Kind: delivery.ForgePull, Action: delivery.PullClosed, Repository: repo, Number: 9, Head: head}); err != nil {
 			t.Fatal(err)
diff --git a/modules/mesh-delivery-indexer/internal/index/seatdefs.go b/modules/mesh-delivery-indexer/internal/index/seatdefs.go
index 3227fcd7..05946ef6 100644
--- a/modules/mesh-delivery-indexer/internal/index/seatdefs.go
+++ b/modules/mesh-delivery-indexer/internal/index/seatdefs.go
@@ -6,8 +6,10 @@ package index
 // it with the format's one reader (mesh-sdk seatdef), and keeps it in its own state `seat-definitions`, keyed by seat
 // and source, as its only writer. Only the trunk's entry is the seat's contract; a pull request's entries go when it
 // closes or merges. A trunk definition that does not parse keeps the trunk's entry and writes the finding
-// seat-definition-unparsed; generated types out of step on a trunk, said by the builder's build end, write the finding
-// generated-types-out-of-step, cleared only by a clean build end of the same contracts module.
+// seat-definition-unparsed; a trunk defining a seat another repository's trunk already defines is refused with the
+// finding seat-defined-twice naming both (a seat has one definition); generated types out of step on a trunk, said by
+// the builder's build end, write the finding generated-types-out-of-step, cleared only by a clean build end of the same
+// contracts module.
 
 import (
 	"context"
@@ -37,9 +39,11 @@ const (
 	// definition's digest changed (ADR 0608 §3).
 	SeatDefinitionChangedEvent = "seat-definition-changed"
 	SeatDefinitionsStream      = "indexer-seat-definitions"
-	// The two findings (ADR 0608 §4, ADR 0564: the indexer writes findings, the condition keeper raises them).
+	// The findings (ADR 0608 §4, ADR 0564: the indexer writes findings, the condition keeper raises them).
 	SeatDefinitionUnparsedKind = "seat-definition-unparsed"
 	OutOfStepKind              = "generated-types-out-of-step"
+	// SeatDefinedTwiceKind: a contracts module's trunk defines a seat another repository's trunk already defines.
+	SeatDefinedTwiceKind = "seat-defined-twice"
 	// The three tools (design 26).
 	SeatDefinitionTool       = "indexer_seat_definition"
 	SeatDefinitionsAgainTool = "indexer_seat_definitions_again"
@@ -284,7 +288,8 @@ func NewSeatDefinitionsAgain(module string, pull int, head, why, by string, at t
 const seatDefinitionsPrefix = "seat-definitions."
 
 // SeatDefinitionsAgain reads a contracts module's seat definitions again now (indexer_seat_definitions_again): its
-// trunk's at the commit the modules state holds it at, or a pull request's at the head given, with the trunk's manifest.
+// trunk's at the commit the modules state holds it at, or a pull request's at the head given, with the head's own manifest
+// (its generated-from as the head has it).
 func (ix *Indexer) SeatDefinitionsAgain(ctx context.Context, module string, pull int, head string) TakenIn {
 	out := TakenIn{Module: module}
 	if ix.SeatDefinitions == nil || ix.ReadFile == nil || ix.Modules == nil {
@@ -315,9 +320,27 @@ func (ix *Indexer) SeatDefinitionsAgain(ctx context.Context, module string, pull
 	if dir == "" {
 		dir = delivery.RootPath
 	}
+	whose := "its trunk's"
+	if pull > 0 {
+		// A pull request is read with its head's own manifest, so a change of generated-from is read as the head has it.
+		file := "module.json"
+		if dir != delivery.RootPath {
+			file = path.Join(dir, file)
+		}
+		raw, err := ix.ReadFile(ctx, m.Repository, head, file)
+		switch {
+		case err != nil:
+			out.Answer = fmt.Sprintf("%s at %.12s cannot be read: %v", file, head, err)
+			return out
+		case raw == nil:
+			out.Answer = fmt.Sprintf("%s#%d's head %.12s holds no %s", m.Repository, pull, head, file)
+			return out
+		}
+		manifest, whose = string(raw), fmt.Sprintf("%s#%d's head's", m.Repository, pull)
+	}
 	cs := contractsModules(map[string]string{dir: manifest})
 	if len(cs) == 0 {
-		out.Answer = module + " is no contracts module: no package artifact of its trunk's manifest names generated-from"
+		out.Answer = module + " is no contracts module: no package artifact of " + whose + " manifest names generated-from"
 		return out
 	}
 	out.Repository, out.Commit, out.Path = m.Repository, m.Commit, m.Path
@@ -348,10 +371,16 @@ func (ix *Indexer) keepTrunk(ctx context.Context, repository, commit string, c c
 	}
 	ix.unparsedFinding(ctx, c.module, unparsed)
 	var errs []error
+	twice := map[string]string{} // seat -> the repository and module whose trunk holds it
 	for seat, d := range defs {
-		errs = append(errs, ix.putDefinition(ctx, SeatDefinitionEntry{Seat: seat, Source: SourceTrunk, Repository: repository,
-			Module: c.module, Directory: c.dir, Commit: commit}, d))
+		holder, err := ix.putDefinition(ctx, SeatDefinitionEntry{Seat: seat, Source: SourceTrunk, Repository: repository,
+			Module: c.module, Directory: c.dir, Commit: commit}, d)
+		if holder != "" {
+			twice[seat] = holder
+		}
+		errs = append(errs, err)
 	}
+	ix.twiceFinding(ctx, repository, c.module, twice)
 	// A seat the module's trunk no longer defines leaves the state.
 	keys, err := ix.SeatDefinitions.Keys(ctx)
 	if err != nil {
@@ -391,7 +420,8 @@ func (ix *Indexer) keepPull(ctx context.Context, repository string, pull int, he
 	for seat, d := range defs {
 		e := base
 		e.Seat = seat
-		errs = append(errs, ix.putDefinition(ctx, e, d))
+		_, err := ix.putDefinition(ctx, e, d)
+		errs = append(errs, err)
 	}
 	for seat, why := range unparsed {
 		e := base
@@ -410,18 +440,25 @@ func (ix *Indexer) keepPull(ctx context.Context, repository string, pull int, he
 	return errors.Join(errs...)
 }
 
-// putDefinition writes one parsed definition at its key, said when its digest moved.
-func (ix *Indexer) putDefinition(ctx context.Context, e SeatDefinitionEntry, d seatdef.Definition) error {
+// putDefinition writes one parsed definition at its key, said when its digest moved. A trunk's definition of a seat
+// another repository's (or module's) trunk already defines is refused, written nowhere: a seat has one definition. The
+// refused one answers who holds it ("<repository> (<module>)").
+func (ix *Indexer) putDefinition(ctx context.Context, e SeatDefinitionEntry, d seatdef.Definition) (string, error) {
 	raw, digest, err := digestOf(d)
 	if err != nil {
-		return err
+		return "", err
 	}
 	e.Definition, e.Digest, e.At = raw, digest, ix.now().UTC()
-	moved := false
+	moved, holder := false, ""
 	err = write(ctx, ix.SeatDefinitions, SeatDefinitionKey(e.Seat, e.Source, e.Repository, e.Pull), func(value []byte, found bool) ([]byte, bool, error) {
 		var cur SeatDefinitionEntry
-		if found && json.Unmarshal(value, &cur) == nil && cur.Digest == digest && cur.Unparsed == "" {
-			moved = false
+		known := found && json.Unmarshal(value, &cur) == nil
+		moved, holder = false, ""
+		if known && e.Source == SourceTrunk && (cur.Repository != e.Repository || cur.Module != e.Module) {
+			holder = cur.Repository + " (" + cur.Module + ")"
+			return nil, false, nil
+		}
+		if known && cur.Digest == digest && cur.Unparsed == "" {
 			return nil, false, nil
 		}
 		moved = true
@@ -431,7 +468,23 @@ func (ix *Indexer) putDefinition(ctx context.Context, e SeatDefinitionEntry, d s
 	if err == nil && moved {
 		ix.sayChanged(ctx, SeatDefinitionChanged{Seat: e.Seat, Source: e.Source, Repository: e.Repository, Pull: e.Pull, Digest: digest})
 	}
-	return err
+	return holder, err
+}
+
+// twiceFinding writes the contracts module's seat-defined-twice finding for the seats its trunk defines that another
+// repository's trunk already holds, naming both, or clears it when there are none.
+func (ix *Indexer) twiceFinding(ctx context.Context, repository, module string, twice map[string]string) {
+	var lines []string
+	for seat, holder := range twice {
+		lines = append(lines, fmt.Sprintf("%s: defined by %s (%s) and already by %s", seat, repository, module, holder))
+	}
+	slices.Sort(lines)
+	ix.finding(ctx, findingID("seat-defined-twice", module), SeatDefinedTwiceKind, len(lines) > 0,
+		fmt.Sprintf("%s's trunk defines seats another trunk already defines, refused: %s", module, strings.Join(lines, "; ")),
+		"A seat on "+module+"'s trunk is already defined by another repository",
+		"A seat has one definition: the one that stood first stays the seat's contract and "+module+"'s is kept nowhere. "+
+			"Remove the seat from one of the two repositories; the next merge of "+module+" clears this.",
+		"No seat on "+module+"'s trunk is defined by another repository.")
 }
 
 // dropPull removes a pull request's entries and its out-of-step files.