[BUGFIX] Don't show duplicates in workspace preview

Fixes workspace filter conditions in case of workspace preview in
t3lib_pageSelect::enableFields().

Additionally cleared up the corresponding comments.

Change-Id: I088928a88cb673f18f218ef691a6c528019317c0
Fixes: #37065
Releases: 6.2, 6.1, 6.0, 4.5
Reviewed-on: https://review.typo3.org/24075
Reviewed-by: Sascha Egerer
Tested-by: Sascha Egerer
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring