Multiple definitions of functions in c

I'm doing a program that uses several functions in separate .h files, however, I've been facing the problem of multiple function definitions because of some includes. I used the code below to instruct the preprocessor to handle this:

#ifndef _HEADER_H_
#define _HEADER_H_
........
........
#endif

However, it does not seem to be working and the linker is creating ambiguities. How do I resolve this issue?