Normally you cannot see it, but if you turn on Ally Nameplates that don't overlapp, you can see multiple NPCs in certain places.

Problem confirmed to be occuring in these areas

The Stormwind City Flight Master (3 spawned)
Heroic Training Dummies in Ironforge and Stormwind City (2 spawned in Stormwind, 3 in Ironforge)
Highlord's Training Dummy in Acherus: The Ebon Hold (2 spawned)
The Rut'theran Village Flight Master (2 spawned)
The Dalaran Flight Master (2 spawned)

These NPCs so far are not affected by the problem

Heroic Training Dummies in Darnassus and The Exodar
The Exodar Flight Master
The Shattrah City Flight Master
The Flight Master in Acherus: The Ebon Hold

A ticket is already in place and "Your ticket will be serviced soon." has been on it for the last hour or so. I hope I get an answer while online and hope the answer doesn't come as soon as I log off.

I'll keep adding more NPCs to the list that I find. Seems to be the dummies and flight masters in certain places so far. This now seems to be a server wide issue or a bug of some kind since it affects flight masters in both Azerotih and Northrend.

Thanks for the report, Kets. Opening an in-game petition was the most appropriate thing to do in this case. Our Game Masters will be sure to look into this as soon as possible.