Dolphins duplicate items when traveling through the nether portal

When a dolphin picks up an item and travels through the nether portal, the item can be found in both dimensions.

Reproduction steps

See also (21w17a) Dolphin item duplication.mp4

1. Download the attached structure a_structure_realizing_it_occasionally.nbt
2. Place the structure file under generated/minecraft/structures of one of your worlds
3. Load the structure
   1. Remove the slime block next to the dispensers and place it next to the piston again
   2. Go through the Nether portal to make sure the Nether is loaded, then go back to the Overworld
4. Press the button on the top dispenser
5. Press the button on the lower dropper
6. Once the dolphin was pushed into the portal enter it as well
   The obsidian item was duplicated

Note that the reproduction steps do not succeed all the time, you might have to try it multiple times (and always make sure to pick up the obsidian items from previous previous tries).