Creating duplicate records in heavily used system

We have written a new application, running in Progress which servesup information from a legacy database.  We have an issue on the live system, where two transactions try to create the same record at the same time.  We're a bit stumped as to the best way round this.

The idea is that the code logs all requests in a UsageHistroy table.  UsageHistory has primary unique key of RequestDate, RequestTime, RequestUniqueID.

The idea is that, for each request to our application, an entry is created, to which information about the transaction is attached (mode, duration etc).

The code is as follows.  Where there is more than one request per second (Date, Time match) we should add a record with the next unique number.  the code is as follows:

            FIND LAST UsageHistory
                WHERE UsageHistory.RequestDate EQ gExecutionStartDate
                  AND UsageHistory.RequestTime EQ gExecutionStartTime
                      NO-LOCK NO-WAIT NO-ERROR.
                         
            IF NOT AVAILABLE UsageHistory THEN
            DO:
                       
                CREATE bufUsageHistory.
               
                ASSIGN bufUsageHistory.RequestDate     = gExecutionStartDate
                       bufUsageHistory.RequestTime     = gExecutionStartTime
                       bufUsageHistory.RequestUniqueID = 1.                   
                       
            END.
            ELSE
            DO:               
                   
                CREATE bufUsageHistory.
       
                ASSIGN bufUsageHistory.RequestDate     = gExecutionStartDate
                       bufUsageHistory.RequestTime     = gExecutionStartTime
                       bufUsageHistory.RequestUniqueID = UsageHistory.RequestUniqueID + 1.
   
            END.

This all works fine in out Dev system, but in Live we keep getting errors such as the following:

** bufPortalUsageHistory already exists with RequestDate 13/04/2011 RequestTime 35589 RequestUniqueID 1. (132)

This must be because of the volume of transactions on the live server, so two processes try to create the same record.  We've tried all sorts of code to try and wrap the above in some sort of transaction, or repeat on fail, but so far no joy.  We don't really have the OpenEdge expertise to come up with the best solution.

Any help would ge greatly appreciated