diff --git a/tests/test_build.py b/tests/test_build.py
new file mode 100644
index 0000000..4dd1bf9
--- /dev/null
+++ b/tests/test_build.py
@@ -0,0 +1,42 @@
+"""Regression: the produced wheel must not contain duplicate zip entries.
+
+The old pyproject.toml had a ``[tool.hatch.build.targets.wheel.force-include]``
+section that re-included `data/` and `_fpforge/data/` already covered by
+``packages = ["src/invisible_playwright"]``. Hatchling wrote every JSON twice
+into the zip; PyPI rejects wheels with duplicate names.
+"""
+from __future__ import annotations
+
+import subprocess
+import sys
+import zipfile
+from collections import Counter
+from pathlib import Path
+
+import pytest
+
+
+@pytest.mark.slow
+def test_built_wheel_has_no_duplicate_entries(tmp_path):
+    """Build the wheel in a clean dir and assert no duplicate zip names."""
+    root = Path(__file__).resolve().parent.parent
+    out = tmp_path / "dist"
+    r = subprocess.run(
+        [sys.executable, "-m", "build", "--wheel", "--outdir", str(out)],
+        cwd=root,
+        capture_output=True,
+        text=True,
+    )
+    assert r.returncode == 0, f"build failed:\n{r.stderr}"
+
+    wheels = list(out.glob("*.whl"))
+    assert len(wheels) == 1, f"expected exactly one wheel, got {wheels}"
+
+    with zipfile.ZipFile(wheels[0]) as zf:
+        names = zf.namelist()
+        dupes = {n: c for n, c in Counter(names).items() if c > 1}
+
+    assert not dupes, f"wheel has duplicate entries (PyPI will reject): {dupes}"
+    # Sanity: the Bayesian data files must still be packaged.
+    json_files = [n for n in names if n.endswith(".json")]
+    assert json_files, "no .json data files in wheel — packaging broken"