There are 73 fossils to collect in Animal Crossing: New Horizons and giving any one of them to Blathers makes him much happier than your latest bug gift. It won't be long though until he's turning down some of the bones you attempt to hand over.

What to do about all these duplicates?

Obviously you could go and immediately sell them at Nooks Cranny but with this doc you now have a quick and easy way to know you had some friends in need of your second T-Rex Skull. I'll leave it up to you to decide if you'll give it to them out of the goodness of your heart or in return for another fossil or bells.