I verified that master on Windows (which requires " instead of ')
> python -c "if object() is 42: pass"
results in the doubled messsage, and that after applying PR 11639 and recompiling, there is only 1 message.

We should test that exactly 1 warning is emitted.  The following fails on master and passes with the parch:

import unittest, warnings

class SyntaxWarningTest(unittest.TestCase):
    def test_syntax_warning_once(self):
        with warnings.catch_warnings(record=True) as w:
            warnings.simplefilter("always")
            compile('if object() is 42: pass\n', '', 'single')
            self.assertEqual(len(w), 1)  # Not 2, see issue 35798

if __name__ == '__main__':
    unittest.main()
    
The original patch added test_comparison_is_literal() in test_grammar.  The 'with' block above could be added at the end.