diff --git a/src/fastapi_oidc_op/store/exceptions.py b/src/fastapi_oidc_op/store/exceptions.py
new file mode 100644
index 0000000..870abb4
--- /dev/null
+++ b/src/fastapi_oidc_op/store/exceptions.py
@@ -0,0 +1,2 @@
+class DuplicateError(Exception):
+    """Raised when a create operation violates a uniqueness constraint."""
diff --git a/tests/test_store/test_exceptions.py b/tests/test_store/test_exceptions.py
new file mode 100644
index 0000000..dd1ec9b
--- /dev/null
+++ b/tests/test_store/test_exceptions.py
@@ -0,0 +1,7 @@
+from fastapi_oidc_op.store.exceptions import DuplicateError
+
+
+def test_duplicate_error_is_exception() -> None:
+    error = DuplicateError("user already exists")
+    assert isinstance(error, Exception)
+    assert str(error) == "user already exists"