Why Multiple Dex files error is thrown

Question

I want to know the reason for the following error: "unable to execute dex: multiple dex files define" .

In my project I have multiple projects having different set of libraries, some projects share same library. What is the reason for this error.

Answer 1

This is because I had two google play services versions pointed in the project. I decoupled the unwanted library and the project compiles.