In some cases, scalac 2.12.9 and 2.13.0 display spurious information that there are plugin duplicates (2.12.8 does not have the issue).

This might be due to the new Plugins.findPluginClassLoader added in the following commit:

scala/scala@cfbb2a2?diff=split#diff-27801265674d0daded4e1af60661525bR71-R98

where the ClassLoader returned by findPluginClassLoader is somehow not "isolated" for detecting plugins available through that classpath.

The following illustrates the issue:

git clone https://github.com/sireum/runtime
runtime/bin/build.cmd compile

For each mill js module, it produces 5 spurious info, but when inspecting one of the modules' plugin classpath:

cd runtime && bin/mill show runtime.test.js.scalacPluginClasspath

Then, there are 6 entries, but only one actually contains the plugin referenced in the spurious information.