This happened to me and happened to my girlfriend these past several weeks. I had a scheduled credit card payment set and then the same payment was scheduled a second time automatically for a different date. I cancelled the duplicate request. Likewise, I had a money transfer set up on Vanguard site to send some money from TD bank to my Vanguard account and it duplicated the transaction a few days after I placed the original transfer, so I ended up sending Vanguard twice as much as I wanted at the time. My girlfriend had a credit card payment doubled this past week.
We called TD bank and they said our computers were buffering and did the duplicate transactions. I think they will not admit any fault on their side. I am scratching my head how the Vanguard money transfer got duplicated.