While Software Engineers are undoubtedly more familiar with the C#### or LNK#### errors generated by Visual Studio, MSB#### errors are more indicative of a problem with the compile or compile environment instead of with the code being compiled.

error MSB6001: Invalid command line switch for "VCBuild.exe". Item has already been added. Key in dictionary: 'tmp' Key being added: 'tmp'

While working on build automation using cygwin, I encountered a number of MSB6001 compile errors. They are generated due to a mismatch between how cygwin, being a linux shell, and the underlying Windows system, handle environment variables. We can see a selection of the currently set environment variables in cygwin:

> env
TEMP=/tmp
temp=path
tmp=path
TMP=/tmp
...

Cygwin automatically sets the TEMP and TMP variables which it treats separate from the temp and tmp variables already set by Windows. nmake, considering environment variables to be case-insensitive, considers them illegally redefined. This can be solved by unsetting cygwin’s automatically defined environment variables:

unset TMP
unset TEMP