diff --git a/prometheus_client/metrics.py b/prometheus_client/metrics.py
index dfa158b9..f50e48ff 100644
--- a/prometheus_client/metrics.py
+++ b/prometheus_client/metrics.py
@@ -1,3 +1,4 @@
+import collections
 import os
 from threading import Lock
 import time
@@ -780,6 +781,10 @@ def __init__(self,
             raise ValueError(f'Overlapping labels for Enum metric: {name}')
         if not states:
             raise ValueError(f'No states provided for Enum metric: {name}')
+        # A repeated state is exposed as two samples with the same labels.
+        duplicates = sorted(s for s, count in collections.Counter(states).items() if count > 1)
+        if duplicates:
+            raise ValueError(f'Duplicate states for Enum metric {name}: {duplicates!r}')
         super().__init__(
             name=name,
             documentation=documentation,
diff --git a/tests/test_core.py b/tests/test_core.py
index 2c29f79a..3a0c9559 100644
--- a/tests/test_core.py
+++ b/tests/test_core.py
@@ -596,6 +596,12 @@ def test_overlapping_labels(self):
         with pytest.raises(ValueError):
             Enum('e', 'help', registry=None, labelnames=['e'])
 
+    def test_duplicate_states(self):
+        registry = CollectorRegistry()
+        with self.assertRaisesRegex(ValueError, r"^Duplicate states for Enum metric e: \['a'\]$"):
+            Enum('e', 'help', states=['a', 'b', 'a'], registry=registry)
+        self.assertEqual(list(registry.collect()), [])
+
     def test_failed_init_does_not_pollute_registry(self):
         registry = CollectorRegistry()
         # A validation failure in __init__ must not leave a half-built collector