diff --git a/docs/roadmap/implementation.md b/docs/roadmap/implementation.md
index a3fe0a7..46ef053 100644
--- a/docs/roadmap/implementation.md
+++ b/docs/roadmap/implementation.md
@@ -1,6 +1,6 @@
 # Performance implementation plan
 
-Status: implementation in progress; Stages 1–5 are complete, Stages 6–11 have not started.
+Status: implementation in progress; Stages 1–6 are complete, Stages 7–11 have not started.
 
 Date: 2026-10-09.
 
@@ -494,7 +494,7 @@ and reusing the opening analysis for framing and aggregate directive handling.
 
 ## Stage 6: install validated registrations without copying the registry
 
-**Status:** not started. **Prerequisite:** Stage 5.
+**Status:** complete. **Prerequisite:** Stage 5.
 
 **Files:** [registry.go](../../registry.go),
 [registry_test.go](../../registry_test.go), registration benchmarks, and evidence.
@@ -519,7 +519,33 @@ and reusing the opening analysis for framing and aggregate directive handling.
 **Completion:** registration copies only pending declarations and preserves
 all-or-nothing validation without rebuilding the existing registry map.
 
-**Evidence:** pending.
+**Evidence:** `GOWORK=off GOTOOLCHAIN=local GOPROXY=off GONOPROXY=none
+GOSUMDB=off go test -mod=readonly -count=1 .` passed. Existing tests cover
+atomic rollback, reserved paths, callback lifecycle, and ownership. Added a
+duplicate-within-batch case and confirmed that it leaves the registry unchanged.
+
+Measurements used Go 1.26.8, linux/amd64, Linux 6.17.0-41-generic, Intel Core
+i5-7260U CPU @ 2.20 GHz, based on source revision `3f8841f` with this stage's
+changes in the working tree. The five-sample command passed:
+`GOWORK=off GOTOOLCHAIN=local GOPROXY=off GONOPROXY=none GOSUMDB=off go test -mod=readonly -run '^$' -bench '^BenchmarkPerformanceRegistration$' -benchmem -benchtime=200ms -count=5 .`.
+Raw output is in `/tmp/commandkit-performance-registration-after-stage6.txt`.
+
+At 8 declarations, individual registration measured 4,873–5,401 ns/op,
+3,152 B/op, and 45 allocations; batch registration measured 4,404–10,155 ns/op,
+3,152 B/op, and 45 allocations (the timing range includes noisy outliers).
+At 128 declarations, individual registration measured 55,274–64,107 ns/op,
+29,399–29,400 B/op, and 293 allocations, compared with the Stage 1 baseline
+414,618–492,261 ns/op, 405,011–405,018 B/op, and 784 allocations. Batch
+registration measured 47,867–53,858 ns/op, 35,969–35,970 B/op, and 296
+allocations, compared with baseline 33,985–36,207 ns/op, 23,013–23,014 B/op,
+and 288 allocations. A slice-only pending check reduced the batch allocation
+cost to about 30.5 KB but was slower (about 61–66 μs) because it scans pending
+names quadratically; the map-based pending index is retained for linear-time
+duplicate validation. The batch regression remains measurable and reflects
+growing the existing map plus maintaining pending entries; restoring a full
+registry rebuild would contradict this stage's selected change. Individual
+registration has a large allocation and latency reduction. No correctness
+issues remain open.
 
 ## Stage 7: cache immutable bridge metadata per app
 
diff --git a/registry.go b/registry.go
index 1bed211..7d12b54 100644
--- a/registry.go
+++ b/registry.go
@@ -9,19 +9,27 @@ func (a *App) Register(commands ...Command) error {
 		return fmt.Errorf("cannot register commands on a nil app")
 	}
 
-	updated := make(map[string]*commandNode, len(a.commands)+len(commands))
-	for name, node := range a.commands {
-		updated[name] = node
-	}
+	pending := make(map[string]*commandNode, len(commands))
 	for _, command := range commands {
 		if err := validateCommandDeclaration(command); err != nil {
 			return err
 		}
-		if _, exists := updated[command.name]; exists {
+		if _, exists := a.commands[command.name]; exists {
 			return fmt.Errorf("command path %q is already registered", command.name)
 		}
-		updated[command.name] = &commandNode{command: cloneCommand(command)}
+		if _, exists := pending[command.name]; exists {
+			return fmt.Errorf("command path %q is already registered", command.name)
+		}
+		pending[command.name] = &commandNode{command: cloneCommand(command)}
+	}
+	if len(pending) == 0 {
+		return nil
+	}
+	if a.commands == nil {
+		a.commands = make(map[string]*commandNode, len(pending))
+	}
+	for name, node := range pending {
+		a.commands[name] = node
 	}
-	a.commands = updated
 	return nil
 }
diff --git a/registry_test.go b/registry_test.go
index 4867e5f..7b38a24 100644
--- a/registry_test.go
+++ b/registry_test.go
@@ -159,6 +159,12 @@ func TestRegisterBatchIsAtomicAndRejectsReservedPaths(t *testing.T) {
 	if _, exists := app.commands["added"]; exists {
 		t.Fatal("failed batch attached a partial registration")
 	}
+	if err := app.Register(added, added); err == nil {
+		t.Fatal("Register accepted a duplicate within a batch")
+	}
+	if _, exists := app.commands["added"]; exists {
+		t.Fatal("failed duplicate batch attached a registration")
+	}
 	for _, name := range []string{"help", "version"} {
 		reserved, err := NewCommand(CommandSpec[struct{}]{Name: name})
 		if err != nil {