According to a comment on wowhead for this item:
I also have both now after buying only one. My theory is that you have to wait out the refund timer and then it grants you the other. I had bought the Wind Rider early on and didn’t have the Gryphon, but now that I returned to get the rest of my tenders, I see that I have it
Teh I didn’t buy either. Blessed be my stinginess
Originally the Spectral mounts and the Alabaster mounts came in a set basically …so if you played both sides you got one of the mounts for each side …I think its bull they were charging to buy each one for each side…
sounds like i need to buy one then.
Try trading it back to the trader NPC I’ve done it before, but I think there is a timer on it for when you can exchange it back.
Asking for a refund on a change that was made in the middle of the month to the trading post seems rather reasonable.
If it was a different month’s trading post and they offered them again but as a bundle I’d probably say it wasn’t a reasonable ask, but every player should get the same priced items for the same month.
650tendies is NOT small. I bought 750tendies not long ago… it came with a free transmog set. 
Tendies should be treated as a RL currency.
send in a ticket to request a refund.
I can confirm, I only bought the windrider and got the gryphon for free.
Nice glad i didnt buy both of them, but a refund is kinda like asking for $20 bucks for Tyrael’s Charger when they put it on TP
FYI: I submitted a ticket regarding getting a refund. The responding GM answered as follows:
“I see you’re looking to check into the mounts Spectral mounts. Both are for sale on Trading Post and it is intended that buying one does not unlock the other factions version. It is a bug that purchasing one unlocks both. This is getting reviewed currently for a fix.”
Condeliesa rice Colin powl fix it already or it’s gonna sound like a gnomergan dish
Heehee.
The “fix” is to either yeet the mounts back from the people who got it for free, or refunding the tenders and leaving it so an unlock of one unlocks both. I wonder which one they’ll go with…