Seems to be an bug / unintended consequence of List Literal, Add List Item, and retaining edits in Play Mode.
Anecdotally feel like this wasn't the case a week or two ago, but don't quote me on that.
1) Have an empty [List of ... Literal]
2) Add a bunch of stuff to it using [Add List Item]
3) Enter and exit Play Mode a bunch of times
4) The List Literal is now full of duplicated items
Customer support service by UserEcho