I've seen previous community questions around the same topic, but yet to find an answer.
We are having issues where one of our forms is creating duplicate leads. The lead is often new, and the lead "Activity log" has the exact same info, at the exact same time. A complete replica (other than the Marketo Unique Code/Marketo Unique Name).
We've also had the same issue when importing through the API.
There's not many of them, but still enough to keep you chasing them and cleaning them up before the numbers get out of hand.