Uploading new cards to a deck, lags, doesn't confirm upload then an hour later I have 3-4x the cards.

This is incredibly frustrating.  If I add cards directly from the +1 card button, upload my picture, name it, it sticks. Ok that's nice. I have 51 additional cards I need to add.  I instead I try the drag and up, upload multiple. Says it took them, says it converted them, I hit create. Literally no cards added.  But you can bet your ass there are images in my library.  So I wait, log out, refresh, no cards added.  I then try to drag the cards from my recent uploads to the multiple card option. Click create. It adds them. I see the card count increasing. Great. Awesome, I am making progress! Then after it's done and I refresh it's got like 300+ cards in there.  What in the hell is going on here? This is not complicated tech here. It's uploading pictures. 

I think your problem is a third party extension, but for the record, the mass card uploader is also still steaming pile of dog poo for people without extensions running.  I found that if you did batches too close together in time(less than a few minutes seperation) that the previous final 5 cards would upload into  the second batch like an echo.  The only way to stop this is to put a good amount of time between each upload.  If you aren't running a conflicting extension check your browser version as the mass uploaders UI is different then all the other uploads inside the session, its actually more like the custom statusmarkers uploader. I feel your pain though bud, I uploaded 500 cards.