I can confirm this error in 2021.2.13f1

Pointed here by Unity QA, because the root issue behind this may be failing Android build for projects pointing to excluded files.
Information from my report:

How to reproduce:
1. Import Path Painter II (current version 2.1.3)
2. Build for Android

Expected results: The project is built successfully
Actual results: "Found plugins with same names" Error is thrown for files excluded from build or the current environment.

Reproducible with:
Probably all currently supported Unity versions are affected. Tested and reproduced with
2021.2.9f1
2020.3.27f1 (LTS)
2019.4.35f1 (LTS)
2018.4.36f1 (LTS)

More info:
Only experienced this building for Android and the cause seems to be that the build includes excluded files in this check. For example, compatibility assemblies (that are constrained to specific Unity versions and environments), but this happens even with Editor only files that supposed to be completely excluded from the build (e.g. under an "Editor" folder).

This only happens when building for Android, so inconsistent behaviour platform to platform regarding a common behaviour. If things like this can happen in Unity, that now becomes a testing nightmare, because now we have to test a lot more things

[Unity versions] x [Platforms] x [RP versions] x [.Net versions] times.

That number quickly runs into the three figures and results in weeks of testing before even the smallest update release.

For example, in our case an asset was built and tested on different platforms earlier. Later the compatibility system was updated to a much better one. It was build-tested, but it was not build tested on all platforms, since general file exclusion from build was not thought to be platform specific. Users now found out the hard way that Android build gets hung up on files that otherwise don't exist in the runtime environment.