Ahh, that would explain why I could not find the thing being defined more than once. I am now staying with my parents for Christmas and cannot really fix this until I get back home in January.
TurfIt - what file was included twice? This is not really a "sabotage" to other compilers from Microsoft - it is just that MSVC does not use makefiles. There have been people building this on Linux with GCC, which I do for my server (although I have not built the latest development branch on my server), so there is not a general problem with Experimental and compilers other than MSVC; however, it is very rare for people to want to use MinGW (people usually use MSVC on Windows or GCC on Linux), so any problems specific to MinGW are hard to track down.
If you let me know the erroneous file, I can give Giuseppe instructions to fix it on his own copy pending me being able to fix it properly when I get back home next month.
Edit: I have just tried compiling with GCC (32-bit) on a very old and slow Linux computer at my parents' house, and it seems to work fine. It is very odd that the problem should be specific to MinGW.