Duplicate Dex Files error (build time error)

A common build time error when using libraries that require Google Play Services is of the form:

Failed on android with com.android.dex.DexException: Multiple dex files...

This error (https://github.com/invertase/react-native-firebase/issues/48) occurs because different versions of google play services or google play services modules are being required by different libraries. The process to fix this is fairly manual and depends on your specific combination of external libraries. Essentially what's required is to check the app level build.gradle file of each external library and establish which ones have a Google Play Services dependency.

You then need to find the lowest common version of each G.P.S module dependency, require that in the app level build.gradle file of your own project, and exclude it from being required by the modules themselves. This will force the use of a consistent version of the G.P.S module.

It's not a good idea to modify the version within the library's build.gradle, as this will be overwritten when you update the library, which will lead to the build breaking again.

A good break down of this process can be found here: https://medium.com/@suchydan/how-to-solve-google-play-services-version-collision-in-gradle-dependencies-ef086ae5c75f