Question Getting error when overwriting existing build

I get this error whenever I overwrite a build, although not when I create a fresh one. Happens with Windows and Android builds.

The same field name is serialized multiple times in the class or its parent class. This is not supported: Base(MonoBehaviour) Output
UnityEditor.BuildPlayerWindow:BuildPlayerAndRun ()

I found several examples of this error when googling, but none of them were regarding a Unity script rather than a user one.