diff --git a/src/hist/axis/__init__.py b/src/hist/axis/__init__.py
index 49a6d03c..1f0281e7 100644
--- a/src/hist/axis/__init__.py
+++ b/src/hist/axis/__init__.py
@@ -253,6 +253,11 @@ def __init__(
         __dict__: dict[str, Any] | None = None,
     ) -> None:
         has_flow = flow if overflow is None else overflow
+
+        categories = list(categories)
+        if len(categories) != len(set(categories)):
+            raise ValueError("Categorical axis entries must be unique")
+
         super().__init__(
             categories,
             metadata=metadata,
diff --git a/tests/baseline/test_image_plot_pull.png b/tests/baseline/test_image_plot_pull.png
index bb3ec178..a9bf1d99 100644
Binary files a/tests/baseline/test_image_plot_pull.png and b/tests/baseline/test_image_plot_pull.png differ
diff --git a/tests/baseline/test_plot1d_auto_handling.png b/tests/baseline/test_plot1d_auto_handling.png
index 1292b2e5..c5aa8f51 100644
Binary files a/tests/baseline/test_plot1d_auto_handling.png and b/tests/baseline/test_plot1d_auto_handling.png differ
diff --git a/tests/test_axis.py b/tests/test_axis.py
index 940c6fc7..32c1271f 100644
--- a/tests/test_axis.py
+++ b/tests/test_axis.py
@@ -64,3 +64,16 @@ def test_axis_disallowed_names():
         hist.Hist(axis.Regular(10, 0, 10, name="sample"))
     with pytest.warns(UserWarning, match="threads is a protected keyword"):
         hist.Hist(axis.Regular(10, 0, 10, name="threads"))
+
+
+def test_duplicate_strcategory():
+    import hist
+
+    try:
+        hist.axis.StrCategory(["a", "a"])
+        raise RuntimeError(
+            "Axis creation should have failed due to duplicate categories"
+        )
+
+    except ValueError:
+        pass