diff --git a/serve/replicas.py b/serve/replicas.py
index 5a139efa8a..6f8406b3d7 100644
--- a/serve/replicas.py
+++ b/serve/replicas.py
@@ -56,11 +56,12 @@ def gpu_groups(cfg: dict) -> list[list[int]]:
             raise ReplicaConfigError(f'"replicas": {spec} does not divide the {len(cards)} cards {cards} evenly: name the '
                                      f'groups ("replicas": [{{"gpus": [0, 1]}}, ...]) or change the count')
         n = len(cards) // spec
-        return [cards[i * n:(i + 1) * n] for i in range(spec)]
-    groups = []
-    for e in spec:
-        g = e.get("gpus") if isinstance(e, dict) else e
-        groups.append(_cards(g))
+        groups = [cards[i * n:(i + 1) * n] for i in range(spec)]
+    else:
+        groups = []
+        for e in spec:
+            g = e.get("gpus") if isinstance(e, dict) else e
+            groups.append(_cards(g))
     if any(not g for g in groups):
         raise ReplicaConfigError('every replica needs its cards: {"gpus": [0, 1]}')
     flat = [c for g in groups for c in g]
diff --git a/serve/test_replicas.py b/serve/test_replicas.py
index 95f1a12e65..00c0c0dfe3 100644
--- a/serve/test_replicas.py
+++ b/serve/test_replicas.py
@@ -27,6 +27,25 @@ def test_named_groups(self):
         self.assertEqual(gpu_groups({"replicas": [{"gpus": [0, 1]}, {"gpus": [2]}]}), [[0, 1], [2]])
         self.assertEqual(gpu_groups({"replicas": [{"gpus": [0, 1]}]}), [])
 
+    def test_automatic_groups_reject_duplicate_cards(self):
+        for cards in ([0, 0], [0, 1, 1, 2], [0, 0, 1, 2], "0,0", "0,1,1,2", "0,00"):
+            with self.subTest(cards=cards):
+                with self.assertRaisesRegex(ReplicaConfigError, "a card is in two replicas"):
+                    gpu_groups({"gpu": cards, "replicas": 2})
+
+    def test_named_groups_still_reject_duplicate_cards(self):
+        for groups in ([[0], [0]], [[0, 1], [1, 2]], [[0, 0], [1, 2]], [[0, 0]]):
+            with self.subTest(groups=groups):
+                with self.assertRaisesRegex(ReplicaConfigError, "a card is in two replicas"):
+                    gpu_groups({"replicas": [{"gpus": g} for g in groups]})
+
+    def test_valid_card_order_and_disabled_replicas_are_unchanged(self):
+        for cards in ([3, 1, 2, 0], "3,1,2,0"):
+            self.assertEqual(gpu_groups({"gpu": cards, "replicas": 2}), [[3, 1], [2, 0]])
+        for count in (None, False, 0, 1):
+            with self.subTest(count=count):
+                self.assertEqual(gpu_groups({"gpu": [0, 0], "replicas": count}), [])
+
     def test_mistakes_are_said(self):
         for cfg in ({"gpu": [0, 1, 2], "replicas": 2}, {"replicas": 2}, {"replicas": "2"},
                     {"replicas": [{"gpus": [0, 1]}, {"gpus": [1, 2]}]}, {"replicas": [{"gpus": []}, {"gpus": [1]}]}):