diff --git a/swh/web/auth/mailmap.py b/swh/web/auth/mailmap.py
--- a/swh/web/auth/mailmap.py
+++ b/swh/web/auth/mailmap.py
@@ -4,6 +4,7 @@
 # See top-level LICENSE file for more information
 
 from django.conf.urls import url
+from django.db import IntegrityError
 from django.db.models import Q
 from django.http.response import (
     HttpResponse,
@@ -40,7 +41,10 @@
     if not request.user.has_perm(MAILMAP_PERMISSION):
         return HttpResponseForbidden()
 
-    UserMailmap.objects.create(user_id=str(request.user.id), **request.data)
+    try:
+        UserMailmap.objects.create(user_id=str(request.user.id), **request.data)
+    except IntegrityError:
+        return HttpResponseBadRequest("This 'from_email' already exists.")
     mm = UserMailmap.objects.get(
         user_id=str(request.user.id), from_email=request.data.get("from_email")
     )
diff --git a/swh/web/tests/auth/test_mailmap.py b/swh/web/tests/auth/test_mailmap.py
--- a/swh/web/tests/auth/test_mailmap.py
+++ b/swh/web/tests/auth/test_mailmap.py
@@ -55,13 +55,31 @@
         api_client,
         reverse("profile-mailmap-list"),
         status_code=200,
-        content_type="application/json"
+        content_type="application/json",
     ).data
     assert len(resp) == 1
     assert resp[0]["from_email"] == "bar@example.org"
     assert resp[0]["display_name"] == "bar"
 
 
+@pytest.mark.django_db(transaction=True)
+def test_mailmap_add_duplicate(api_client, mailmap_user):
+    api_client.force_login(mailmap_user)
+
+    check_api_post_response(
+        api_client,
+        reverse("profile-mailmap-add"),
+        data={"from_email": "foo@example.org", "display_name": "bar"},
+        status_code=200,
+    )
+    check_api_post_response(
+        api_client,
+        reverse("profile-mailmap-add"),
+        data={"from_email": "foo@example.org", "display_name": "baz"},
+        status_code=400,
+    )
+
+
 @pytest.mark.django_db(transaction=True)
 def test_mailmap_endpoints_error_response(api_client, mailmap_user):
     api_client.force_login(mailmap_user)