Performance PowerBi. What is the most optimal?

Maybe this can help for the question 3.

Form A have one Form B associate. 1 to 1 relation. 
Both of them have a User. BUT users between they can be different.
In my experience i can do this in 3 ways.
1- With a loose table, then in the Forms tables i can use a calculate column, with something like this: Caculate(users_table(user_name), user_table(id) = FormA/B(user_id))
2- With duplicate table, then userA is related with FormA. And userB is related with FormB.
3- With one user table and related with both of then, BUT one of the relation should be inactive.

If i understand correctly AllisonKennedy you said that if i need 2 filters in mi visuals that are for users ( in this case we need that ) the correct way will duplicate the table.
I understand that the user table generally is "small", this option make sense.

Migasuke I don't know if i understand correctly, but you said that how my source is a SQL database literally is "almost" the same because query folding is optimiced for this source?
I understand that if we can use the source to ETL process we should use that, but is irrelevant in this case?

Thanks so much both.