diff --git a/src/aegis_gitea_mcp/auth.py b/src/aegis_gitea_mcp/auth.py
index fb0c57f..7598b93 100644
--- a/src/aegis_gitea_mcp/auth.py
+++ b/src/aegis_gitea_mcp/auth.py
@@ -9,6 +9,7 @@ token to the :class:`~aegis_gitea_mcp.identity.Identity` a tool call runs as.
 
 from __future__ import annotations
 
+import asyncio
 import hashlib
 import hmac
 import time
@@ -103,6 +104,7 @@ class AdminIdentityResolver:
         self._executor = executor
         self._login: str | None = None
         self._retry_at = 0.0
+        self._lookup_lock = asyncio.Lock()
 
     async def resolve(self, access: AccessToken | None) -> Identity:
         """Return the admin identity, or raise for an access token of another kind."""
@@ -113,9 +115,12 @@ class AdminIdentityResolver:
             raise ToolError("admin-token mode needs GITEA_TOKEN, which is not configured")
         token = secret.get_secret_value()
         if self._login is None and time.monotonic() >= self._retry_at:
-            self._login = await self._lookup_login(token)
-            if self._login is None:
-                self._retry_at = time.monotonic() + _LOGIN_RETRY_SECONDS
+            # One lookup for concurrent first calls: later callers wait, then see the result.
+            async with self._lookup_lock:
+                if self._login is None and time.monotonic() >= self._retry_at:
+                    self._login = await self._lookup_login(token)
+                    if self._login is None:
+                        self._retry_at = time.monotonic() + _LOGIN_RETRY_SECONDS
         return Identity(login=self._login, mode="admin-token", token=token)
 
     async def _lookup_login(self, token: str) -> str | None:
diff --git a/tests/test_auth.py b/tests/test_auth.py
index 747b174..b6aed0b 100644
--- a/tests/test_auth.py
+++ b/tests/test_auth.py
@@ -17,8 +17,13 @@ from aegis_gitea_mcp.auth import (
     StaticBearerVerifier,
 )
 from aegis_gitea_mcp.errors import ToolError
-from aegis_gitea_mcp.gitea import GiteaExecutor
-from tests.test_server_helpers import API, BEARER_TOKEN, GITEA_TOKEN, server_settings
+from aegis_gitea_mcp.gitea import GiteaExecutor, ToolOutcome
+from tests.test_server_helpers import (
+    API,
+    BEARER_TOKEN,
+    GITEA_TOKEN,
+    server_settings,
+)
 
 # Static bearer verifier
 
@@ -171,3 +176,23 @@ async def test_missing_gitea_token_is_refused() -> None:
 
     with pytest.raises(ToolError, match="GITEA_TOKEN"):
         await resolver.resolve(None)
+
+
+async def test_concurrent_first_calls_send_one_login_lookup() -> None:
+    import asyncio
+
+    lookups: list[str] = []
+
+    class SlowExecutor(GiteaExecutor):
+        async def request(self, method: str, path: str, *args: Any, **kwargs: Any) -> ToolOutcome:
+            lookups.append(path)
+            await asyncio.sleep(0.01)  # a real network round trip yields to other calls
+            return ToolOutcome(False, {"data": {"login": "admin"}}, 200)
+
+    settings = server_settings()
+    resolver = AdminIdentityResolver(settings, SlowExecutor(settings))
+
+    identities = await asyncio.gather(*(resolver.resolve(None) for _ in range(10)))
+
+    assert {identity.login for identity in identities} == {"admin"}
+    assert lookups == ["/user"]