1) Person 1 sends some bitcoin to Person 2 with a deliberately small fee so it never gets confirmed.
2) The bitcoin eventually gets returned to Person 1 as the fee was too low.
I've seen some people on another forums calling it double spending, but I'm pretty sure that's not what it is.
no, that is not double spending. it is just an unconfirmed transaction which has always been unsafe to accept (considered as payment being made for instance). also remember that when a transaction is made and broadcast to the network, technically it can remain out there forever. for instance the receiver can store it locally and keep broadcasting it until it either is confirmed or becomes invalid. so technically it could never "gets returned to person 1".
In addition to simply rebroadcasting... the receiver could actually execute a CPFP (Child Pays For Parent) transaction that spends the UTXO from the unconfirmed transaction with a large fee to push the original transaction through.
As others have said, unless the sender attempts to also spend the coins in a second transaction, it isn't double spending, it's just a really bad scam attempt