Thanks for the report and the PR (and sorry for the delay in responding).  There were several problems with building on macOS without installed header files, which is no longer supported in the most recent Xcode / Command Line Tools releases, as well as a problem specific to 2.7 which caused such builds to not find the system zlib.  All of these issues were addressed in the changes for bpo-36231 (c421c66a58a6caae30f0679d7e61411418e67cec) and bpo-19960 (da7f6dbbf52fe7074360a9eae918806da60139c5) which were merged to 2.7 a few months ago and are being released with 2.7.17. With these changes, the double-join problem your PR was correcting no longer occur.