I think it depends on which version of Maya you're using, but it likes to incorrectly flip normals when dealing with -1 scaled objects, but only if you try to manually freeze or flip normals back before combining them with other objects.
Try simply duplicating with a -1 scale as usual, then Mesh > Combine with the original. No other operations in between. Normals should automatically flip and display correctly and you shouldn't get any issues when exporting.
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Try simply duplicating with a -1 scale as usual, then Mesh > Combine with the original. No other operations in between. Normals should automatically flip and display correctly and you shouldn't get any issues when exporting.