diff --git a/lib/mpl_toolkits/tests/test_axes_grid.py b/lib/mpl_toolkits/tests/test_axes_grid.py
index 58358b686651..0ff4f2b013b6 100644
--- a/lib/mpl_toolkits/tests/test_axes_grid.py
+++ b/lib/mpl_toolkits/tests/test_axes_grid.py
@@ -12,10 +12,16 @@
 # The original version of this test relied on mpl_toolkits's slightly different
 # colorbar implementation; moving to matplotlib's own colorbar implementation
 # caused the small image comparison error.
-@pytest.mark.parametrize("legacy_colorbar", [False, True])
-@image_comparison(['imagegrid_cbar_mode.png'],
+@image_comparison(['imagegrid_cbar_mode.png', 'imagegrid_cbar_mode.png'],
                   remove_text=True, style='mpl20', tol=0.3)
-def test_imagegrid_cbar_mode_edge(legacy_colorbar):
+def test_imagegrid_cbar_mode_edge():
+    # Note, we don't use @pytest.mark.parametrize, because in parallel this
+    # might cause one process result to overwrite another's.
+    for legacy_colorbar in [False, True]:
+        _test_imagegrid_cbar_mode_edge(legacy_colorbar)
+
+
+def _test_imagegrid_cbar_mode_edge(legacy_colorbar):
     mpl.rcParams["mpl_toolkits.legacy_colorbar"] = legacy_colorbar
 
     X, Y = np.meshgrid(np.linspace(0, 6, 30), np.linspace(0, 6, 30))