Longstanding Teleport visual bug

Teleporting to a target that an ally is already teleporting to causes your own TP warning/arrival particle to fail to appear, leading to a "stealthiport" situation. It's been around for a while, but with URF's flash/TP meta, it's fresh in my mind again.

Repro steps:

1. Player 1 teleports to any target - ward, turret, minion, it doesn't matter. Teleport warning particle appears on target.
2. Player 2 - allied with Player 1 - waits a second or two, then casts teleport on the same target as Player 1. No visible change to target.
3. Player 1's teleport completes as normal. Warning particle vanishes, Player 1 arrives at target.
4. Player 2's teleport completes as normal. With no warning particle to announce the spell, Player 2 shows up completely by surprise.

This can absolutely be exploited for player advantage because it allows 2v1 ambush situations where it's completely impossible for the enemy team to anticipate the 2nd player's arrival. You know how you can generally prepare a trap or CC for a teleporting player? You can't do that when his buddy shows up, and you have no way of knowing you're about to walk into a 1v2 (or more!) instead of a 1v1.