This is a really isolated issue at the moment that I haven't been able to replicate (I can sometimes) so I haven't quite gotten to the point of making a github ticket for those reasons.
Essentially the working theory is that if you load an admin module editor page, and then navigate away from that page in a way that causes the active browser to suspend the memory for that tab (or something), when you come back to resume that browser tab, it causes a rehydration event or something which is causing blocks that were originally there to be duplicated.
Workflow on this image:
1. Added Key Features and Column Callout blocks, saved page
2. Came back and reloaded the page, added product info slider, resource download, and product reviews
3. Navigated away for a while to get some more information
4. Came back to find blocks had duplicated
I'm still not sure if this is exactly the problem as it has been so intermittent, so any thoughts at all on whether I'm even going down the right path to try and figure this error out would be really helpful.