Hi Mono,
you should start the join without validation and limit access. I am not really sure, which are your settings..
The database join element with a validation rule (isUnique) for a text field is probably not practical, because user can write names in different ways.
You should start from the beginning...
customer table: id, customer (field) >>> are customers = jusers?
adress table: id, customer (dbjoin, id as value, customer as label)
If you set now the validation (isUnique) to customer it works, because it checks the customer_id. Users can not select same customer twice.
It's easier to understand, where you want to go, if you write down the structure of your tables.