diff --git a/modules/claude-licence-manager/cmd/claude-licence-manager/keyform.go b/modules/claude-licence-manager/cmd/claude-licence-manager/keyform.go
new file mode 100644
index 000000000..46732bd3b
--- /dev/null
+++ b/modules/claude-licence-manager/cmd/claude-licence-manager/keyform.go
@@ -0,0 +1,41 @@
+package main
+
+// A licence's key form (design 39 bus-and-switch §10, B6): the name every verb and tool takes and answers, written
+// in the characters a key of the bus takes. The key is the store's alone.
+
+import (
+	"context"
+	"strings"
+)
+
+// KeyFor is a licence's key: its name in lower case, each `@` written as `_at_`, and every other character
+// outside letters, digits, `-` and `.` written as `_`.
+func KeyFor(name string) string {
+	var b strings.Builder
+	for _, r := range strings.ToLower(name) {
+		switch {
+		case r == '@':
+			b.WriteString("_at_")
+		case r >= 'a' && r <= 'z', r >= '0' && r <= '9', r == '-', r == '.':
+			b.WriteRune(r)
+		default:
+			b.WriteByte('_')
+		}
+	}
+	return b.String()
+}
+
+// keyTaken names the licence other than name whose key is name's, or "" when none is.
+func (m *Manager) keyTaken(ctx context.Context, name string) (string, error) {
+	all, err := m.Store.Licences(ctx)
+	if err != nil {
+		return "", err
+	}
+	key := KeyFor(name)
+	for _, l := range all {
+		if l.Name != name && KeyFor(l.Name) == key {
+			return l.Name, nil
+		}
+	}
+	return "", nil
+}
diff --git a/modules/claude-licence-manager/cmd/claude-licence-manager/keyform_test.go b/modules/claude-licence-manager/cmd/claude-licence-manager/keyform_test.go
new file mode 100644
index 000000000..29e0e36c3
--- /dev/null
+++ b/modules/claude-licence-manager/cmd/claude-licence-manager/keyform_test.go
@@ -0,0 +1,72 @@
+package main
+
+import (
+	"context"
+	"strings"
+	"testing"
+	"time"
+)
+
+// design 39 bus-and-switch §10, B6.
+func TestALicencesKeyForm(t *testing.T) {
+	for name, key := range map[string]string{
+		"jane@example.org":                     "jane_at_example.org",
+		"Jane.Doe@Example.ORG":                 "jane.doe_at_example.org",
+		"0c1d2e3f-4a5b-6c7d-8e9f-a0b1c2d3e4f5": "0c1d2e3f-4a5b-6c7d-8e9f-a0b1c2d3e4f5",
+		"team key_1":                           "team_key_1",
+		"a+b/c:d":                              "a_b_c_d",
+		"ops-key":                              "ops-key",
+	} {
+		if got := KeyFor(name); got != key {
+			t.Errorf("KeyFor(%q) = %q, want %q", name, got, key)
+		}
+	}
+}
+
+// design 39 bus-and-switch §10: an adoption whose key another licence has is refused, in the audit and as an event.
+func TestAnAdoptionWhoseKeyIsTakenIsRefused(t *testing.T) {
+	ctx := context.Background()
+	t.Run("an API key", func(t *testing.T) {
+		mm := newMesh(t)
+		_ = mm.store.SaveLicence(ctx, Licence{Name: "Ops-Key", Kind: "api-key"})
+		_, err := mm.m.adoptKey(ctx, "ops-key", "sk-ant-"+strings.Repeat("a", 40), "a test")
+		if err == nil || !strings.Contains(err.Error(), "Ops-Key") {
+			t.Fatalf("adopted a key whose key form is taken: %v", err)
+		}
+		if l, _ := mm.store.Licence(ctx, "ops-key"); l != nil {
+			t.Fatal("the clashing licence was kept")
+		}
+		assertRefused(t, mm)
+	})
+	t.Run("a login", func(t *testing.T) {
+		mm := newMesh(t)
+		_ = mm.store.SaveLicence(ctx, Licence{Name: "ACCT-1@example.org", Kind: "api-key"})
+		r := mm.login("laptop", "rt-laptop", true, t0.Add(-time.Minute))
+		adopted, err := mm.m.Consider(ctx, []Holdings{r})
+		if err != nil || len(adopted) != 0 {
+			t.Fatalf("adopted %v (%v), want none", adopted, err)
+		}
+		if len(mm.vendor.exchanged) != 0 {
+			t.Fatalf("a refused login was exchanged: %v", mm.vendor.exchanged)
+		}
+		if o, _ := mm.store.Outcome(ctx, Fingerprint("rt-laptop")); o != Refused {
+			t.Fatalf("outcome %q, want refused", o)
+		}
+		assertRefused(t, mm)
+	})
+}
+
+func assertRefused(t *testing.T, mm *miniMesh) {
+	t.Helper()
+	said := false
+	for _, e := range mm.events {
+		said = said || strings.HasPrefix(e, "licence.refused ")
+	}
+	audited := false
+	for _, a := range mm.store.Audits {
+		audited = audited || a["what"] == "refused"
+	}
+	if !said || !audited {
+		t.Fatalf("refusal said %v, audited %v: %v", said, audited, mm.events)
+	}
+}
diff --git a/modules/claude-licence-manager/cmd/claude-licence-manager/manager.go b/modules/claude-licence-manager/cmd/claude-licence-manager/manager.go
index fe8b208cd..1fa28f126 100644
--- a/modules/claude-licence-manager/cmd/claude-licence-manager/manager.go
+++ b/modules/claude-licence-manager/cmd/claude-licence-manager/manager.go
@@ -348,6 +348,24 @@ func (m *Manager) adoptOne(ctx context.Context, c Candidate, reports []Holdings)
 		return "", nil
 	}
 
+	// A new licence whose key another licence already has is refused before its grant is exchanged
+	// (design 39 bus-and-switch §10).
+	if held, err := m.Store.LicenceForAccount(ctx, c.Identity.AccountUUID); err != nil {
+		return "", err
+	} else if held == nil {
+		taken, err := m.keyTaken(ctx, NameFor(c.Identity))
+		if err != nil {
+			return "", err
+		}
+		if taken != "" {
+			why := "its key " + KeyFor(NameFor(c.Identity)) + " is the key of the licence " + taken
+			_ = m.Store.RecordOutcome(ctx, c.Fingerprint, c.Node, c.Identity.AccountUUID, Refused, why)
+			_ = m.Store.Audit(ctx, "refused", map[string]any{"node": c.Node, "licence": NameFor(c.Identity), "reason": why})
+			_ = m.Emit("licence.refused", map[string]any{"node": c.Node, "account": NameFor(c.Identity), "reason": why})
+			return "", nil
+		}
+	}
+
 	// Adopting is refreshing (ADR 0206 §4): the exchange is the check, and from here this module is the
 	// only holder of a live refresh token for the account.
 	r := m.Vendor.Refresh(ctx, offered)
@@ -751,6 +769,16 @@ func (m *Manager) adoptKey(ctx context.Context, name, key, from string) (map[str
 	if held != nil && held.Kind != "api-key" {
 		return nil, fmt.Errorf("%s is a subscription; an API key needs a name of its own", name)
 	}
+	taken, err := m.keyTaken(ctx, name)
+	if err != nil {
+		return nil, err
+	}
+	if taken != "" {
+		why := "its key " + KeyFor(name) + " is the key of the licence " + taken
+		_ = m.Store.Audit(ctx, "refused", map[string]any{"licence": name, "kind": "api-key", "from": from, "reason": why})
+		_ = m.Emit("licence.refused", map[string]any{"licence": name, "from": from, "reason": why})
+		return nil, fmt.Errorf("%s is refused: %s", name, why)
+	}
 	l := Licence{Name: name, Kind: "api-key", Sealed: m.Crypt.Seal(key), AdoptedAt: m.Now().UnixMilli()}
 	if err := m.Store.SaveLicence(ctx, l); err != nil {
 		return nil, err