The only duplicates I keep on my island are craftable dragons. I battle for parts to create one, usually get it quickly and then have extra parts due to a tourney or Tales task. I then collect more parts to craft another one so I can use up the parts. I keep them around so I can use evolve them for points and then trade them for jewels since we need them more and more in Tales crafting deco tasks.

I have a few commons in storage in case I ever run into the 'breed with a red type dragon' situation again. Breeding Fire with Fire gives you an outcome of like 10 seconds. However, since I no longer do Bingo, I could probably let them go too.

The last exception I have is one baby Angel. I used it to get all the AI hybrids now can't seem to let it go I will though, as soon as I need that spot in a habitat.