Add assertion to check whether duplicate history store inserts are modifies

We have some build failures related to duplicate values being inserted into the history store. At the moment, we've only ever seen this happening with full updates flagged with the WT_UPDATE_RESTORED_FROM_DS flag.

This won't cause a visibility issue since it is a full update however, reinsertions of modifies could cause problems. We should add an additional assert to check whether the duplicate insert is a modify to better assess the severity of this bug.