[BUGFIX] Prevent duplicate page tree items when entry points intersect

In a scenario, where a backend user has two entry
points, in which the second one intersects the first
one (is a direct sub-page), pages were shown
multiple times.

The reason behind this was, that inside
PageTreeRepository->groupAndSortPages() every page
was added regardless if it was already added in a
previous run. This is the case, when the same pages
are found in different depth levels.

E.g.

1 - Home (DB mount 1)
  2 - Page 1
  2 - Page 2 (DB mount 2)
    3 - Subpage 1 // will be added for level 2 and 3
    3 - Subpage 2

To prevent this, a guard clause was added, which
checks whether the page was already added by
comparing the uid value.

Resolves: #96512
Resolves: #97774
Related: #96367
Releases: main, 11.5, 10.4
Change-Id: I65439011b894f05faef9cb507d905b3d7d3b709f
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75223


Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>