Python 2.7's setup.py has incorrect behaviour when adding the SDKROOT to the beginning of the include path while searching.

When searching paths, find_file first checks is_macosx_sdk_path  to see if it needs to add the sdk_root: https://github.com/python/cpython/blob/2.7/setup.py#L87-L88

This is mostly correct, except one of the path prefixes it checks is /Library: https://github.com/python/cpython/blob/2.7/setup.py#L64

The Xcode CLT path is located in /Library, so this check passes. That means the /Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk portion of the path gets repeated, leading to an invalid path.

I recognize Python 2.7 isn't in active development, but I have a minimal patch to fix this that I will be submitting.

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.