General :: Using Makefile - 'Multiple Definition' Compile Errors

As part of an assignment, our class was given a small set of c++ classes which are compiled using a makefile. As part of the assignment, I have created a namespace and saved in it a .cpp file. The functions, constants and typedefs from the namespace are now used in one of the original files, and so I added an <#include MyFile.cpp> to the file I modified, in addition to a using MyNamespace;

When I try to use the original makefile, the compiler gives a lot of errors about multiple definitions of every function in the namespace. What do I need to do to resolve this? Do I need to change the makefile or one of the files themselves? I am new to Linux programming and makesfiles.