diff --git a/browser_cli/cli.py b/browser_cli/cli.py
index 6b063a5..63afe2a 100755
--- a/browser_cli/cli.py
+++ b/browser_cli/cli.py
@@ -21,7 +21,13 @@ from browser_cli.commands.dom import dom_group
 from browser_cli.commands.extract import extract_group
 from browser_cli.commands.session import session_group
 from browser_cli.commands.search import search_group
-from browser_cli.client import send_command, BrowserNotConnected, REGISTRY_PATH, display_browser_name
+from browser_cli.client import (
+    send_command,
+    BrowserNotConnected,
+    REGISTRY_PATH,
+    active_browser_targets,
+    display_browser_name,
+)
 
 console = Console()
 
@@ -51,6 +57,30 @@ NATIVE_HOST_DIRS = {
 }
 
 
+def _rename_target_profile(target_browser: str | None) -> str | None:
+    if target_browser:
+        return target_browser
+
+    active = active_browser_targets()
+    if len(active) == 1:
+        return active[0].profile
+    return None
+
+
+def _ensure_unique_browser_alias(alias: str, target_browser: str | None) -> None:
+    target_profile = _rename_target_profile(target_browser)
+
+    profiles: dict[str, str] = {}
+    if REGISTRY_PATH.exists():
+        try:
+            profiles = json.loads(REGISTRY_PATH.read_text())
+        except Exception:
+            profiles = {}
+
+    if alias in profiles and alias != target_profile:
+        raise click.ClickException(f"Browser alias '{alias}' already exists")
+
+
 def _native_host_wrapper_path() -> Path:
     if sys.platform == "darwin":
         base_dir = Path.home() / "Library" / "Application Support" / "browser-cli"
@@ -184,6 +214,7 @@ main.add_command(clients_group)
 def cmd_clients_rename(target_browser, alias):
     """Set the profile alias used to identify this browser instance."""
     try:
+        _ensure_unique_browser_alias(alias, target_browser)
         send_command("clients.rename_profile", {"alias": alias}, profile=target_browser)
     except BrowserNotConnected as e:
         console.print(f"[red]Error:[/red] {e}")
diff --git a/extension/manifest.json b/extension/manifest.json
index fd9b582..0d6def5 100644
--- a/extension/manifest.json
+++ b/extension/manifest.json
@@ -1,7 +1,7 @@
 {
   "manifest_version": 3,
   "name": "browser-cli",
-  "version": "0.5.6",
+  "version": "0.5.7",
   "description": "Control your browser from the terminal via browser-cli",
   "permissions": [
     "tabs",
diff --git a/pyproject.toml b/pyproject.toml
index 05f970e..d7d37bd 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -1,6 +1,6 @@
 [project]
 name = "browser-cli"
-version = "0.5.6"
+version = "0.5.7"
 description = "Control your real running browser from the terminal via a Chrome extension"
 requires-python = ">=3.10"
 dependencies = [
diff --git a/tests/test_cli.py b/tests/test_cli.py
index 09d4ee7..039b509 100644
--- a/tests/test_cli.py
+++ b/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"])