[TASK] Show duplicated entrypoints in site module as warning

If an entrypoint URL is used more than once this will lead in errors and
a warning will be shown in the site module.

Resolves: #87533
Releases: master
Change-Id: I2ed7e8859685ee28392599448b79107d3db2d6cc
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63681


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Frank Nägler <frank.naegler@typo3.org>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Frank Nägler <frank.naegler@typo3.org>