Multiple permissions can be added to project for individual user

[das-Abroxas]

Describe the bug

You can add multiple permissions to a project for an individual user which makes it impossible to guess which permission should be used for requests send by the specific user.

To Reproduce

Steps to reproduce the behavior:

1. Create a project
2. Add multiple different permissions with AddUserToProjectRequest

Expected behavior

Only one unique permission may exist for a user per project so that it can be correctly checked whether the specific user is allowed to perform the desired action or not.