Okay md_5, thanks a lot for researching. We can code some workarounds when using the event in our own plugins if we need to use potions so it wont be a big problem. Thank you for your work!

Yeah what I said on IRC — we have to trigger the LEFT_CLICK_AIR event based on the arm swing animation because the client has no other packet for it. So this can't be fixed without removing LEFT_CLICK_AIR entirely which would cause greater strife than the two events.

Can confirm,

The player's hand moves twice when you click/break

So is this behaviour a vanilla issue?
I discovered something new. The event is being called twice with the same hand (not offhand). The only difference is that first the action is RIGHT_CLICK_AIR, and the second time the action is LEFT_CLICK_AIR (and I just right clicked the air with the potion)

It behaves the same if you are in creative or in survival (except when I throw a splash potion in survival mode). I checked with both events and:

• If I consume the potion: first it fires the PlayerInteractEvent twice, and then it fires the PlayerItemConsumeEvent (This one behaves exactly the same whether I am in creative or survival)
• If I use a splash potion: it fires one PlayerInteractEvent,, then it fires ProjectileLaunchEvent, then it fires PlayerInteractEvent again! (I am using only one splash potion) ; If I am in survival mode, it just calls PlayerInteractEvent once, if I am in creative, twice.

Also another thing, the PlayerInteractEvent only happens twice when I am aiming right clicking the air, not if I am right clicking a block or a mob.

If the client is sending two packets for the same hand, it's not possible to distinguish the two events.
Have a look at other events, like PlayerItemConsumeEvent or ProjectileLaunchEvent.

Please also test if the client behaves differently in creative and survival mode.

My code already returns the event if it is the OffHand. I thought you meant another thing with "each hand", here is the code of the test plugin I attached and the one using in the video Iink I put in the description:

    @EventHandler
    public void PlayerInteractEvent(PlayerInteractEvent e) {
        if (e.getHand() == EquipmentSlot.OFF_HAND)
            return;
        e.getPlayer().sendMessage("You are interacting with: " + e.getItem().getType().toString());
        if (e.getHand() == EquipmentSlot.HAND)
            e.getPlayer().sendMessage("Your HAND called the event");
    }

I'm confused, you're already using PlayerInteractEvent.getHand().

Is there a way we can check which hand was? When I use e.getHand I can compare with HAND or OFF_HAND

Spigot calls those events for each packet received by the client. So it seems the client is sending one packet for each hand here now, too.