When you VOID a SOP transaction, the transaction should be moved to the SOP history tables.
The header record in SOP10100 moves to SOP30200
The lines records in SOP10200 move to SOP30300 etc
We have noticed problems in the past if the process is not completed.
When you void a SOP transaction, if you then CLOSE the "Sales Transaction Entry" window, the VOID posting will then complete and you will see it create a "Sales Voided Posting Journal" report.
From your description above, it looks like your process has not completed, because those records should not exist in SOP10100 or SOP10200 anymore.
You should check to see if there are entries for the voided back order in the SOP30200 and SOP30300. If they are there, you should clean out the remaining "partial" entries from SOP10100 and SOP10200.
(a SOP reconcile may clean these incomplete record for you)