Steps to reproduce:
1. Create a new project
2. Export the project to a preferred folder
3. Export the same project a second time to the same folder

Expected result: exporting project to the same location should not throw the following errors

Trying to add file .../libs/unity-classes.jar to the list of ouptut files in the build report, but a file at that path has already been added.
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)
Trying to add file .../src/main/AndroidManifest.xml to the list of ouptut files in the build report, but a file at that path has already been added.
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)

Reproduced in: 2019.1.0a7, 2018.2.11f1, 2018.1.9f2, 2018.1.0a3
No repro in: 2018.1.0a2, 2018.1.0a1, 2017.4.14f1
Regression since: 2018.1.0a3

Fixed in: 2019.1.0a10