diff --git a/prometheus_client/openmetrics/parser.py b/prometheus_client/openmetrics/parser.py
index 3e285093..50fa3419 100644
--- a/prometheus_client/openmetrics/parser.py
+++ b/prometheus_client/openmetrics/parser.py
@@ -139,9 +139,12 @@ def _parse_labels_with_state_machine(text):
             if char == '\\':
                 state = 'labelvalueslash'
             elif char == '"':
-                if not METRIC_LABEL_NAME_RE.match(''.join(labelname)):
-                    raise ValueError("Invalid line: " + text)
-                labels[''.join(labelname)] = ''.join(labelvalue)
+                ln = ''.join(labelname)
+                if not METRIC_LABEL_NAME_RE.match(ln):
+                    raise ValueError("Invalid line, bad label name: " + text)
+                if ln in labels:
+                    raise ValueError("Invalid line, duplicate label name: " + text)
+                labels[ln] = ''.join(labelvalue)
                 labelname = []
                 labelvalue = []
                 state = 'endoflabelvalue'
@@ -217,6 +220,10 @@ def _parse_labels(text):
             # Replace escaping if needed
             if "\\" in label_value:
                 label_value = _replace_escaping(label_value)
+            if not METRIC_LABEL_NAME_RE.match(label_name):
+                raise ValueError("invalid line, bad label name: " + text)
+            if label_name in labels:
+                raise ValueError("invalid line, duplicate label name: " + text)
             labels[label_name] = label_value
 
             # Remove the processed label from the sub-slice for next iteration
diff --git a/tests/openmetrics/test_parser.py b/tests/openmetrics/test_parser.py
index 81873388..63dd5e42 100644
--- a/tests/openmetrics/test_parser.py
+++ b/tests/openmetrics/test_parser.py
@@ -560,6 +560,11 @@ def test_invalid_input(self):
             ('a{a="1"b="2"} 1\n# EOF\n'),
             ('a{a="1",,b="2"} 1\n# EOF\n'),
             ('a{a="1",b="2",} 1\n# EOF\n'),
+            # Invalid labels.
+            ('a{1="1"} 1\n# EOF\n'),
+            ('a{a="1",a="1"} 1\n# EOF\n'),
+            ('a{1=" # "} 1\n# EOF\n'),
+            ('a{a=" # ",a=" # "} 1\n# EOF\n'),
             # Missing value.
             ('a\n# EOF\n'),
             ('a \n# EOF\n'),