[BUGFIX] Duplicate model property names should not be valid

If a model has a proerty and a relation with the same name
it would result in a class with 2 properties of the same name

This patch also improves the client side validation before saving

Resolves: #50185
Releases: 6.2
Change-Id: I087773fd238fd0ac446868966aa12789e7103992
Reviewed-on: https://review.typo3.org/29666
Reviewed-by: Nico de Haen
Tested-by: Nico de Haen