It sounds as if you did redeclaration by include, like:

File Preparx.for
==========================
INCLUDE "Serverstuff.for"
SUBROUTINE Preparx()
...
END

INCLUDE statement basically works as a copy/paste macro, and in effect you define Subroutine ServerStuff in every of your sources. I don't think that C/Fortran has anything to do with that error.

I don't know how it compiled with another compiler, but that's not the way to do things in Fortran. Actually, that's not how to do things in C either -- C include files should contain just function prototypes, not their bodies.

If my analysis is correct, you should get rid of those INCLUDE lines. Have in mind, though, that I haven't seen your actual code and that I might easily have misdiagnosed the situation.