Right clicking water while duel wielding spawn eggs creates two of the mobs

The bug

When you right click water surface or lava surface while holding a spawn egg in both hands, you can spawn two entities with one click. This is inconsistent with the behaviour when doing the same thing against a block, where only the egg in the main hand is used.

The bug

1. Hold any spawn eggs in the two slots
2. Right click lava or water
   → You should get 2 types of entities, or 2 of 1 type of entity