[BUGFIX] Fix select with multiple enabled not being saved correctly

If the 'multiple' field in the fieldConfig is set the
$newDatabaseValueArray is returned as is, with duplicate entries. If this
is not the case $newDatabaseValueArray is returned as before, being
modified with the array_unique function.

Resolves: #76918
Releases: master, 7.6
Change-Id: Id8ddac95162a929aeb1b5d7bae33a460317b46cb
Reviewed-on: https://review.typo3.org/49981


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Karol Lamparski <klamparski@gmail.com>
Tested-by: Karol Lamparski <klamparski@gmail.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>