I just bought a couple of CSC gift cards from the shop. Both times I got this message:

Oh No! Looks like the page refreshed. You are seeing this message to prevent you from accidentally making a duplicate purchase.

Even though I'm sure I only clicked once. However the purchase went through correctly anyway, and the items were in my inventory and the correct amount of CSC deducted from my balance. It seems like it's just an issue with the error message displaying erroneously instead of getting the message saying I purchased the item.

It has still occasionally been happening in some of the other special shops (like Quest Shop, Millionaire Center, etc.) but not nearly as often as it was when the lag was really bad. It had been happening like nearly every purchase, now it's just once in a while. At least for me personally.