A little trick I learned a while ago:

in one module only
#define MAIN

in the .h file

#ifdef MAIN
#define U8 unsigned char
...
...
#else
#define U8 extern unsigned char
...
...
#endif

U8 a_variable;

Now you have 'intern' and 'extern' defined the same place and no mistakes (in this regard :-) )are possible.

Have fun,

Erik