I posted this in the feedback app, but I'm also gonna post it here in case anyone has any solutions.
I upgraded my Lumia 1020 from Windows 8.1 to build 10572 - my SMS messages were intact, as expected.
I reset my phone, now my messages would not sync, as has previously been reported.
I upgraded from 572 to 581 and all of my messages did eventually sync, but my pre-572 messages were now doubled.
Resetting my phone again did not fix this.
I downgraded back to 8.1 briefly and all of my pre-572 messages synced without doubling, but none of my messages from 572 and 581 synced.
I upgraded from 8.1 to 586.11 - messages from my downgrade to 8.1 were intact, and messages from 572 and 581 were restored from sync, but my older pre-572 messages still synced doubled.
Resetting again did not fix - all messages synced, and messages from the recent downgrade did not sync doubled, but messages from before my initial upgrade to 10572 are still doubled
I upgraded my Lumia 1020 from Windows 8.1 to build 10572 - my SMS messages were intact, as expected.
I reset my phone, now my messages would not sync, as has previously been reported.
I upgraded from 572 to 581 and all of my messages did eventually sync, but my pre-572 messages were now doubled.
Resetting my phone again did not fix this.
I downgraded back to 8.1 briefly and all of my pre-572 messages synced without doubling, but none of my messages from 572 and 581 synced.
I upgraded from 8.1 to 586.11 - messages from my downgrade to 8.1 were intact, and messages from 572 and 581 were restored from sync, but my older pre-572 messages still synced doubled.
Resetting again did not fix - all messages synced, and messages from the recent downgrade did not sync doubled, but messages from before my initial upgrade to 10572 are still doubled
