Windows Headers for Cygwin/MinGW

In trying to get my mingw configuration working again, I reinstalled the cygwin toolchain several times in various configurations. Along the way, I noticed that there are two distinct sets of Windows headers. The original set (from Cygwin b20.1) has several win*.h files which all "#include <windows.h>" and windows.h then includes several files in the Windows32 subdirectory. Things work fine when I use this set of headers. The second set (included in the cygwin-inst-*.tar.gz files) is intended to better mimic the MS Windows headers. When I use these headers, I get complaints about various things being redefined. The build does not complete. I suspect that starting with Cygwin v1.0 and continuing with all subsequent commercial and net releases the included set will be the one which mimics the MS Windows headers. Is there a way to detect which set the compiler is using? Is it important to accomodate both sets of headers? Craig