I have spent an inordinate amount of time this year thinking I have been Whammed when I just catch a certain part of a pop song while not being aware of the song being on.
That song is Good Luck, Babe! by Chappell Roan. Towards the end it starts to slow down and if you're not listening to the rest of the song and just turn the radio on to thr last few bars, it sounds extremely similar to the opening few notes of Last Christmas.
3:18 through this. If you yune your brain out and listen to that bit, assuming you've just turned the radio on to that bit, it sounds very similar to the first few notes of Last Christmas. Or at least that's where my brain has been for the last few months this song has been on the radio.