Interesting Problems with the Compiler and Linker

[Nuggets08]

Alright, I was programming and found the need to use a function pointer, I programmed it alright, my syntax was right, but the linker said it was being declared multipally. Well, I found a different way to program it, and just went on. I had this same problem trying to declare variables in header files too(I declared the function pointer in the header too).
Can anyone shed any light as to why it's complaining about this? I am really bad at figuring out linker problems, I have a mental block or something with them.