What appears to have happened is that the customer wanted a new phone and the AT&T representative told them that the fastest way would be to change numbers. They said that porting in the older Mint Mobile number would delay when the new phone could be handed over.
The customer, wishing to get their hands on a new iPhone 16 as soon as possible, agreed and decided to get a new AT&T number instead. But after the first bill showed up it turned out that the poster now had two lines to pay for: one on the new AT&T number and another on the older Mint Mobile number.
The bill is charging the customer activation fees for both lines as well as for the new iPhone as if the new phone wasn’t part of the trade-in deal. It’s a nightmare scenario and it gets worse.

This all might just be human error. | Image credit — PhoneArena
It’s also unknown whether this whole ordeal happened because of deliberate action on the representatives’ part or whether this was just human error. The takeaway is clear however: if you’re trading in your phone at AT&T you’ve now got another thing to be wary of. Always double and triple check what you’ve signed up for before walking out of the store.