Ah.The reason is, that the Pro version switches from purchase reporting through the browser pixel to purchase reporting through the Google Measurement Protocol. The Measurement Protocol is much more accurate for new orders (for orders from the moment you activate the pro version).
But the way I implemented it, the purchase hit through the Measurement Protocol will happen on each order status change towards order completion (only one report). That means it also happens on orders before activation of the Pro version.
Since on those old orders the purchase report already happened through the pixel, the new report through the Measurement Protocol leads to a duplication.
If you don’t have too many order status changes on old orders anymore, then keep the Pro version running. The duplicate reportings will go away.
I will think of a way to prevent this entirely. It is tricky, so it might take a few days.