The first forums part of the URL is the installation directory.
Here for example it is community/threads/friendly-url-question.29485/
The second forums part of the URL is when viewing an actual forum.
For example: community/forums/xenforo-questions-and-support.25/
So if the installation directory was forums instead of community, forums would appear like so:
forums/forums/xenforo-questions-and-support.25/