fix that alias can only exist once and when dublicate give error

tests/test_cli.py

@@ -45,6 +45,27 @@ def test_clients_rename_uses_global_browser_target_when_set():
     send_command.assert_called_once_with("clients.rename_profile", {"alias": "work"}, profile=None)
     assert "Restart the browser" not in result.output
 
+def test_clients_rename_rejects_duplicate_alias(tmp_path):
+    registry_path = tmp_path / "registry.json"
+    registry_path.write_text('{"work": "/tmp/work.sock", "old-id": "/tmp/old-id.sock"}', encoding="utf-8")
+
+    with patch("browser_cli.cli.REGISTRY_PATH", registry_path), patch("browser_cli.cli.send_command") as send_command:
+        result = CliRunner().invoke(main, ["clients", "rename", "--browser", "old-id", "work"])
+
+    assert result.exit_code != 0
+    assert "Browser alias 'work' already exists" in result.output
+    send_command.assert_not_called()
+
+def test_clients_rename_allows_same_alias_for_same_target(tmp_path):
+    registry_path = tmp_path / "registry.json"
+    registry_path.write_text('{"work": "/tmp/work.sock"}', encoding="utf-8")
+
+    with patch("browser_cli.cli.REGISTRY_PATH", registry_path), patch("browser_cli.cli.send_command") as send_command:
+        result = CliRunner().invoke(main, ["clients", "rename", "--browser", "work", "work"])
+
+    assert result.exit_code == 0
+    send_command.assert_called_once_with("clients.rename_profile", {"alias": "work"}, profile="work")
+
 def test_install_help_lists_supported_browsers():
     result = CliRunner().invoke(main, ["install", "--help"])