diff --git a/pandas/core/indexes/multi.py b/pandas/core/indexes/multi.py
index 48cf6000d100d..3d149eccc746b 100644
--- a/pandas/core/indexes/multi.py
+++ b/pandas/core/indexes/multi.py
@@ -3503,12 +3503,8 @@ def equal_levels(self, other: MultiIndex) -> bool:
 
     def _union(self, other, sort) -> MultiIndex:
         other, result_names = self._convert_can_do_setop(other)
-        if (
-            any(-1 in code for code in self.codes)
-            and any(-1 in code for code in other.codes)
-            or other.has_duplicates
-        ):
-            # This is only necessary if both sides have nans or other has dups,
+        if other.has_duplicates:
+            # This is only necessary if other has dupes,
             # otherwise difference is faster
             result = super()._union(other, sort)
 
diff --git a/pandas/tests/frame/methods/test_combine_first.py b/pandas/tests/frame/methods/test_combine_first.py
index ad6122501bc19..f7da28a43590d 100644
--- a/pandas/tests/frame/methods/test_combine_first.py
+++ b/pandas/tests/frame/methods/test_combine_first.py
@@ -398,11 +398,7 @@ def test_combine_first_string_dtype_only_na(self, nullable_string_dtype):
             pa_version_under7p0 and nullable_string_dtype == "string[pyarrow]",
         ):
             df2.set_index(["a", "b"], inplace=True)
-        with tm.maybe_produces_warning(
-            PerformanceWarning,
-            pa_version_under7p0 and nullable_string_dtype == "string[pyarrow]",
-        ):
-            result = df.combine_first(df2)
+        result = df.combine_first(df2)
         with tm.maybe_produces_warning(
             PerformanceWarning,
             pa_version_under7p0 and nullable_string_dtype == "string[pyarrow]",