It’s legal, and even common
This is what happens when people don’t resolve their problem on time. Instead of just one, you now have three negative items on your report.
The first account is the original default from AT&T. They have charged-off the account and sold it to a collection agency. The original charge-off should have a $0 balance.
The first collection agency have probably done nothing to collect (or did they?) and sold it to a second collection company. This collection account should also have a $0 balance.
There should be just one account with the original balance, and that would be the collection agency who currently owns your debt. They are the agency you should deal with.
You should be able to get a settlement for 25% lump sum or less. The second agency probably bought your debt for few cents on the dollar, so they will be making a nice profit even at 25% (See how-to-pay-off-debt.html for more information).
All of the three negative accounts weigh heavily on your credit score. They will fall off 7-1/2 from date of first deficiency, whether paid, settled or unpaid. After two years their impact on your credit score will diminish.
Spare your time and skip the dispute. It won’t remove any of the items since they are legitimate. Don’t believe all that advice you may find in the internet about disputing every negative item on your report. It’s not true. The items may be removed for a short period of time if the collection agencies respond late, but eventually they will return onto your report.
Next time deal with your debts in time and you will suffer less damage.