If you defeat an ender dragon for the first time, then summon a dragon using an end crystal, and then defeat that dragon, for some reason the amount of experience you get is the same, and it also drops more dragon eggs. (Normally, only one dragon egg can be obtained per world, but this method makes it possible to obtain two or more dragon eggs.
Does this only happen on a realm, or does the issue occur in a local world?
Are you using any add-ons? If so, which ones (name and creator), and does the issue still occur if those are removed?