I added ProtocolLib to the Test plugin to prove that the issue is serverside, not clientside.
Here is whatis happening:
- The minecraft client sends packets based on these conditions:
- If you press rightclick while holding anything in hand, a BlockPlace packet will be sent, regardless of whether the item in hand is a block or not
- If you press rightclick while facing a block, a UseItem packet will be sent, regardless of whether you hold an item in hand or not
- If you click a block while holding an item, both packets will be sent in the following order: UseItem(Main Hand), BlockPlace(If item in Main Hand), UseItem(Off Hand), BlockPlace(If Item in OffHand)
- Let's assume the player is holding an item in the main hand and clicks a block. The packets will be sent in the order provided above.
- Spigot first evaluates the Packets in order, so the UseItem event will be processed first and fires a PlayerInteractEvent at PlayerInteractManager:492
- After this Spigot evealuates this packet, the next packet will be the BlockPlace packet. The relevant part of the Method is at PlayerConnection:926-966 Spigot does some raycasts here to check if the player is clicking a block or not, to prevent 'rogue armswings'. Normally this check would be successfull if the player clicked a block. However this fails, since we teleported the player beforhand. Because of that Spigot assumes the player clicked the air and fires a PlayerInteractEvent of the type RIGHT_CLICK_AIR.
And so we get 2 PlayerInteractEvents from the same action.
This could be fixed by checking whether the PlayerInteractManager already handled the event before checking the movingobjectposition
I hope this explanation was usefull 
To test this for yourself, check the repo mentioned in the OP
Cheers
Max
Interacts are fired purely on what the client does.