If you search the Internet for "windows can't duplicate display" you'll find plenty of people with the same issue you're describing going back years. I don't think it's the TV, although I did find one person that had to turn off "theater mode" on his Sony TV to get his display to duplicate, so I suppose it's possible that there's a TV setting that affects it.
I went to 'Settings', then 'Display' then scrolled down to 'Multiple displays' and chose 'Duplicate these displays' it went back to normal. Not sure how long it will last though, but it did go back to normal.
