Edit: After not noticing until this race and being informed in the comments, I decided to find out what’s behind this phenomenon.
Here’s an article about it.
I also wanted to know when those double chequered flags started, so I had a look at both 2025 and 2024 race finishes, and as far as I could determine, the first time this occurred was at Abu Dhabi 2024. Every other race in 2024 looked to only have the official flag.