Welcome to the board. When I was on Signy in the mid 1980's, there was a famous "enforced winter" not so long beforehand - can't remember when, more than 5 years, less than 10 probably - when last call couldn't get in and so anyone due out on last call had an extra winter. There were a small number of people who had already done 2 and so that was their 3rd along with summer only and people who had done 1 winter.
Also, around the same sort of time, one of the East European countries had a base on the peninsula or maybe King George Island, Polish I think, who couldn't afford to send a supply ship down one summer(!). They decided the men had enough supplies and so they all had an extra winter too! It wasn't so bad for them as the other bases in the vicinity took pity and passed them various goodies.
It's certainly very unusual and you're the only one I've heard of who did it voluntarily.