diff --git a/checks/manifests/main.go b/checks/manifests/main.go
index 676daa90..5d456637 100644
--- a/checks/manifests/main.go
+++ b/checks/manifests/main.go
@@ -1,5 +1,5 @@
 // Package manifests reads every module's manifest as the catalogue's tree holds it and refuses what no judge could
-// read: a module.json that does not decode, and two manifests that name one module.
+// read: a module.json that does not decode, a module.json giving one field twice, and two manifests that name one module.
 //
 // It judges no manifest (novox/hq ADR 0578 §2): the checker judges every change's manifests with the indexer's judge, the
 // one judge, so the field, seat, bus and catalogue rules are said once, there. Until 2026-11 this check parsed each
@@ -9,6 +9,8 @@
 package manifests
 
 import (
+	"bytes"
+	"encoding/json"
 	"fmt"
 	"os"
 	"path/filepath"
@@ -18,7 +20,9 @@ import (
 )
 
 // Problems is what is wrong with the tree's manifests under root/modules, one line each: a module.json whose bytes are
-// no manifest (catalogue.Decode: not JSON, or a field of the wrong shape), then a module named by two manifests.
+// no manifest (catalogue.Decode: not JSON, or a field of the wrong shape), one giving a field twice (a decoder keeps the
+// last and drops the first without a word: a merge of two branches that each added the field writes it so), then a module
+// named by two manifests.
 func Problems(root string) ([]string, error) {
 	files, err := filepath.Glob(filepath.Join(root, "modules", "*", "module.json"))
 	if err != nil {
@@ -38,6 +42,10 @@ func Problems(root string) ([]string, error) {
 			out = append(out, fmt.Sprintf("%s: %v", rel, err))
 			continue
 		}
+		if twice := givenTwice(raw); twice != "" {
+			out = append(out, fmt.Sprintf("%s: the field %s is given twice; a judge reads only the last", rel, twice))
+			continue
+		}
 		if named[m.Module] {
 			out = append(out, fmt.Sprintf("%s: %s was already given; two manifests name one module", rel, m.Module))
 			continue
@@ -46,3 +54,47 @@ func Problems(root string) ([]string, error) {
 	}
 	return out, nil
 }
+
+// givenTwice is the first field an object of raw gives twice, as its path (`invokes`, `build.artifacts[0].env`), or empty.
+func givenTwice(raw []byte) string {
+	d := json.NewDecoder(bytes.NewReader(raw))
+	var walk func(path string) string
+	walk = func(path string) string {
+		t, err := d.Token()
+		if err != nil {
+			return ""
+		}
+		switch t {
+		case json.Delim('{'):
+			seen := map[string]bool{}
+			for d.More() {
+				k, err := d.Token()
+				if err != nil {
+					return ""
+				}
+				key, _ := k.(string)
+				at := key
+				if path != "" {
+					at = path + "." + key
+				}
+				if seen[key] {
+					return at
+				}
+				seen[key] = true
+				if twice := walk(at); twice != "" {
+					return twice
+				}
+			}
+			_, _ = d.Token()
+		case json.Delim('['):
+			for i := 0; d.More(); i++ {
+				if twice := walk(fmt.Sprintf("%s[%d]", path, i)); twice != "" {
+					return twice
+				}
+			}
+			_, _ = d.Token()
+		}
+		return ""
+	}
+	return walk("")
+}
diff --git a/checks/manifests/main_test.go b/checks/manifests/main_test.go
index 252fa559..90be3b25 100644
--- a/checks/manifests/main_test.go
+++ b/checks/manifests/main_test.go
@@ -10,7 +10,7 @@ import (
 )
 
 // The tree's manifests are ones a judge can read, and no module is named twice. A row is a catalogue of its own: a
-// manifest that is no JSON and two manifests of one module are refused, naming the file; a manifest with a field this
+// manifest that is no JSON, one giving a field twice and two manifests of one module are refused, naming the file; a manifest with a field this
 // check does not know is not (it judges nothing: the checker does, ADR 0578 §2). The catalogue itself passes.
 func TestEveryManifestDecodesAndNamesOneModule(t *testing.T) {
 	for _, row := range []struct {
@@ -21,6 +21,10 @@ func TestEveryManifestDecodesAndNamesOneModule(t *testing.T) {
 		{name: "a manifest the judge can read", files: map[string]string{"a": `{"module":"a","version":"1"}`}},
 		{name: "a field no part knows is the judge's to refuse", files: map[string]string{"a": `{"module":"a","version":"1","unheard":true}`}},
 		{name: "bytes that are no manifest", files: map[string]string{"a": `{"module":`}, refused: "modules/a/module.json"},
+		{name: "a field given twice", files: map[string]string{"a": `{"module":"a","version":"1","invokes":["x.y"],"build":{"artifacts":[{}]},"invokes":["z.w"]}`},
+			refused: "modules/a/module.json: the field invokes is given twice"},
+		{name: "a field given twice deeper", files: map[string]string{"a": `{"module":"a","version":"1","build":{"artifacts":[{"name":"t","name":"u"}]}}`},
+			refused: "the field build.artifacts[0].name is given twice"},
 		{name: "two manifests of one module", files: map[string]string{"a": `{"module":"x","version":"1"}`, "b": `{"module":"x","version":"1"}`},
 			refused: "two manifests name one module"},
 	} {