During Squidward's insane quest to get his clarinet back, two Alternate-Universe Squidward appear in the mirrors on his left and right and torture him by playing their clarinets loudly.
"Squid Plus One"
Alternate-Universe Squidward appears several times during the episode. He appears whenever Squidward looks into something reflective. AU Squidward is the person Squidward takes to the Galleria Diphtheria as Squidward made SpongeBob carry a mirror. In the end, AU Squidward tells the real Squidward he will always be his plus one, which delights the latter.