Many 15A receptacles are 20A pass-through, so you should not be able to overload it without tripping the protection circuit, even when it's on a 20A circuit.
You may have more than one receptacle that is fed by two circuits.
First, you should be able to eliminate any that only have one cable attached.
On any that have two cables (or more), remove one hot lead from each receptacle. Turn power back on...see if the breaker trips. If it doesn't, turn things back off and one by one, attach the removed hot lead back. Turn power on, if it trips, break the tab. Continue through the rest of them in case there's a second one that needs the tab broken. Once you've reached the final one, power should stay on in both circuits.
It is possible that one wire run feeds multiple receptacles...if so, you'll have to break the tab on more than one of them.
Once you've fixed things so they all stay on and all ports work, you should either replace that breaker with a dual one, or buy the handle that connects them together so if one side trips, it shuts the other one off, too. Otherwise, someone turning one breaker off may end up thinking all power is off in that box, and find out they only got half of it...it could be quite a shock.
If you have GFCI protection, to ensure things work properly, when you have to break one tab, you should probably break the neutral tab too...but, make sure the hot and neutral from a single cable are not crossed with the other one feeding the box...eg., if the top receptacle is being fed, make sure it's companion neutral is attached to the neutral side of the same receptacle.