ASP application Error/valid handling with Repository pattern

Question

The Action: Create New User

 

Controller - calls repository Create

 

in the Create method you edit if the user id is already in database. If it is you want to avoid an insert and returnthe error

 

ModelSate cannot be updated to signal the controller an error 

 

once the validation is moved from the controller what is the best pattern to use.

 

in the above scenario a new Repository can be implemented 'DoesUserExist' to return the user name in the event the user id already exist.

This is a simple edit - but what if the validation is complex? How would one implement a robust validation process using the repository pattern