There is an inconsistency in re.sub() when substituting at the end of a string using a prior match with a '*' qualifier: the substitution now occurs twice.  For example:

txt = re.sub(r'\s*\Z', "\n", txt)

This should work like txt.rstrip() + "\n", but beginning in 3.7, the re.sub version now matches twice and changes any non-empty whitespace into "\n\n" instead of "\n". (If there is no trailing whitespace it only matches once.)

The bug is the same if '$' is used instead of '\Z', but it does not happen if an actual character is specified (e.g. a substitution of r'\s*x' does not substitute twice if x has preceding whitespace).

I tested 2.7.17, 3.6.9, 3.7.7, 3.8.2, and 3.9.0a4, and it starts to fail in 3.7.7 and beyond.

Attached is a test program.