want to add unique ID field while signup help plz

I want to add a unique ID/driver license/ social security type field in signup process. So only user who have that ID will be able to signup. This ID will be registered in database and won't allow multiple user IDs. Any idea how to implement the process? 

Just add a new field in the fields builder and check the unique box.

Anything more advanced will require custom work.

I am having problem here. i have chosen type:number  so when i want to put the max value to 999999999 it is not saving it, instead putting 254. The IDs i am going to use is of 13 digit length. 

A question if i create the ID field in join form, how to duplicate the block in edit profile so the user be able to edit it.