I've mused that a solid "delayed clipboard clear" feature would do some kind of atomic Compare-And-Swap, because otherwise the originating program could conceivably blow away something it didn't put there.
The clipboard lib you're using just spawns xclip (or xsel) on Unix. xclip has a -loops argument that is the maximum number of pastes. So you could just spawn:
xclip -loops 1 -in -selection clipboard
There are clipboard manager apps that could grab the paste immediately and persist it, but I'm not sure how common they are or if they are default anywhere.