Trying to Fix Primary Key Conflict Before Merge

I have a situation where I have a DataTable that is initially constructed from a SQL Server table.  In SQL Server the Primary Key is a single field called "Index_No", which has its Identity Specification set to 'Yes" and 'Identity Increment' set to 1.
Imagine upon the start of a new session that the DataTable has 100 records loaded in it from the DB.  For simplicity's sake, let's assume that the Index_No values of these records range from 1 to 100.
Further imagine that during the current session, User A adds 5 new records.  So the records would be assigned Index_No values of: 101, 102, 103, 104, & 105.  Important: Until the user presses 'Save', the records strictly remain only in the DataTable; they're not yet saved back to the DB.
At any point, the user can check whether other users have entered new records too.  Let's assume that User B started off his session a few seconds after User A.  In his work, User B created two new ...