[Solved] two transaction IDs for the same purchase

This seems to be invisible to our users, but in our web database where we back up users' purchases, we are seeing a small percentage with near-duplicate records. What is happening is they have two backups with different transaction IDs (and sometimes different payloads). We don't seem to have corresponding reports of anyone getting double-charged. What generates the transaction ID and could cause this?

To sum up...
- These are non-consumable items (can buy only once).
- Same product ID backed up twice.
- The transaction IDs are different.
- The payloads are sometimes the same, sometimes not.
- ProcessPurchase possibly being called multiple times, only seconds apart.
- This is only confirmed for iOS currently.
- Users are apparently not being double-charged.
- This is a very small percentage of transactions so far.

[edited because I discovered that payloads are not always the same]

 

Could this be transactions being restored? Apple can give different transaction ids when non consumable purchases are restored.

 

That could very well be the case. I didn't know those could have different tx ids. Thanks!