Debugging duplicate phase executions ...

I have a simple Maven project (a web-app) that uses the '
frontend-maven-plugin'
to handle various Javascript-related chores (installing node, running
Grunt, Karma etc.)

Unfortunately, the frontend stuff runs twice on a 'mvn clean site' command
- when
I turned on debug-output (in Eclipse) I can "see" the reactor list 2 calls
to the frontend plugin.

Is there any techniques or help anyone can suggest to try to figure out
what is causing
the extra execution?

Thanks in advance,

Mike Norman