Yeah, it happens a lot, and it's really hard to resist. I have digital folders full of notes, character designs, setting lore etc. for so many different comics, more than I could ever create.
I'd say, make all the notes, and then keep them around. You can always combine the best bits and characters from a bunch of different ones into one really cool idea later, or integrate them into your current comic. The main thing is to try not to get distracted and start doing too many comics at once!