Hi Peter, thanks so much for your reply.
We encounter this problem on many different hosted feature layers, it hasn't just been a problem that we've encountered on a single service, so I'm not sure that republishing will resolve the issue.
In all cases where we see this problem, we have all editing options enabled (add/delete/update for attributes and geometry).
We have tried changing various settings in the Editor widget, but regardless of the settings we have still been getting duplicate polygons in our data.
Thanks for any further advice!