Tiger Exchange desk check: the phone number from step three is what the desk looks up, and if a Tigerexch ID already sits on it, no second one gets opened.
So what happens when someone asks for a tiger exchange new id while an older one already sits on their number? The desk says no. If you have lost track of the old username, the useful step is to ask for help with that ID rather than requesting a fresh one, because a second Tigerexch ID will not be opened on that number however the request is worded.
Different number, different story. A family member with their own phone can apply for their own Tiger Exchange ID in the normal way; the rule follows the mobile number, not the household.
Why does the rule exist at all? Mostly so one person does not end up holding three half-used accounts with balances scattered across them, which makes every later withdrawal question slower to answer for both sides (and, in practice, it makes the desk's own records far easier to keep straight).