Option 'onlyOnce' should validate only once, even when set to false

[ravage84]

Currently when the option "onlyOnce" is set to false, the behavior adds the validation rule to each field of a group.
This leads to multiple calls of multColumnUniqueness, which means the same data base query is done multiple times.
Optimally this should be prevented by validating once but setting the error message to each unique field in the valiation error array.

[dereuromark]

I agree. No need to invoke the validation x times, only display x times then :)