diff --git a/backend/app/api/routes/users.py b/backend/app/api/routes/users.py
index 1748f58484..b13e5006bf 100644
--- a/backend/app/api/routes/users.py
+++ b/backend/app/api/routes/users.py
@@ -2,6 +2,7 @@
 from typing import Any
 
 from fastapi import APIRouter, Depends, HTTPException
+from sqlalchemy.exc import IntegrityError
 from sqlmodel import col, delete, func, select
 
 from app import crud
@@ -65,7 +66,15 @@ def create_user(*, session: SessionDep, user_in: UserCreate) -> Any:
             detail="The user with this email already exists in the system.",
         )
 
-    user = crud.create_user(session=session, user_create=user_in)
+    try:
+        user = crud.create_user(session=session, user_create=user_in)
+    except IntegrityError:
+        # Another request created this email between the check and the insert
+        session.rollback()
+        raise HTTPException(
+            status_code=400,
+            detail="The user with this email already exists in the system.",
+        )
     if settings.emails_enabled and user_in.email:
         email_data = generate_new_account_email(
             email_to=user_in.email, username=user_in.email, password=user_in.password
@@ -155,7 +164,15 @@ def register_user(session: SessionDep, user_in: UserRegister) -> Any:
             detail="The user with this email already exists in the system",
         )
     user_create = UserCreate.model_validate(user_in)
-    user = crud.create_user(session=session, user_create=user_create)
+    try:
+        user = crud.create_user(session=session, user_create=user_create)
+    except IntegrityError:
+        # Another request created this email between the check and the insert
+        session.rollback()
+        raise HTTPException(
+            status_code=400,
+            detail="The user with this email already exists in the system",
+        )
     return user
 
 
diff --git a/backend/tests/api/routes/test_users.py b/backend/tests/api/routes/test_users.py
index 9c4cdd5991..0a12034c4e 100644
--- a/backend/tests/api/routes/test_users.py
+++ b/backend/tests/api/routes/test_users.py
@@ -162,6 +162,24 @@ def test_create_user_existing_username(
     assert "_id" not in created_user
 
 
+def test_create_user_email_taken_between_check_and_insert(
+    client: TestClient, superuser_token_headers: dict[str, str]
+) -> None:
+    # Two requests with the same email can both pass the check. The unique
+    # index stops the second insert, which must still answer 400.
+    data = {"email": settings.FIRST_SUPERUSER, "password": random_lower_string()}
+    with patch("app.crud.get_user_by_email", return_value=None):
+        r = client.post(
+            f"{settings.API_V1_STR}/users/",
+            headers=superuser_token_headers,
+            json=data,
+        )
+    assert r.status_code == 400
+    assert (
+        r.json()["detail"] == "The user with this email already exists in the system."
+    )
+
+
 def test_create_user_by_normal_user(
     client: TestClient, normal_user_token_headers: dict[str, str]
 ) -> None:
@@ -355,6 +373,22 @@ def test_register_user_already_exists_error(client: TestClient) -> None:
     assert r.json()["detail"] == "The user with this email already exists in the system"
 
 
+def test_register_user_email_taken_between_check_and_insert(
+    client: TestClient,
+) -> None:
+    # Two signups with the same email can both pass the check. The unique
+    # index stops the second insert, which must still answer 400.
+    data = {
+        "email": settings.FIRST_SUPERUSER,
+        "password": random_lower_string(),
+        "full_name": random_lower_string(),
+    }
+    with patch("app.crud.get_user_by_email", return_value=None):
+        r = client.post(f"{settings.API_V1_STR}/users/signup", json=data)
+    assert r.status_code == 400
+    assert r.json()["detail"] == "The user with this email already exists in the system"
+
+
 def test_update_user(
     client: TestClient, superuser_token_headers: dict[str, str], db: Session
 ) -> None: