Multiple definitions of "xxxx"
Included files are included before compilation, really replacing the #include with the content of the file.

If you write int DebugX = 1; you define a variable. And it is defined in every source file which includes it. Then the linker finds it in more then one unit and ends with "multiple definitions" error.

The variable must be defined only in one compilation unit (.c, .cpp). For example in your case customFunctions.cpp. To make it available in other source files put extern int DebugX; into the .h file.