When you insert a row to the database, a single row is inserted. The database doesn't duplicate the row automatically in any case. Possible reasons for the duplication include at least:
- you execute the statement twice: Try to find if you for example have the call to ExecuteNonQuery mistakenly twice in your code
- the insert statement is included twice in your SQL string. Using debugger, see if the insert statement is mistakenly put to the SQL string twice.
- the method in your code is called twice. This should be easy to pinpoint. Put a breakpoint on the method and see when it's hit
- you have a trigger that's causing the duplication. Investigate the triggers of the table using SSMS
- a procedure does the insertion: see if you have a procedure that inserts the other row and if it's called from your application (or from a trigger)