[FEATURE] add uniqueCheck for comments

Resolves: #54361
Releases: 6.1, 4.7, 4.5
Change-Id: I0ac1cf3e8aac470587085be7878fd3bb9c768d71
Reviewed-on: https://review.typo3.org/31742
Reviewed-by: Ralf Zimmermann <ralf.zimmermann@tritum.de>
Tested-by: Ralf Zimmermann <ralf.zimmermann@tritum.de>