Hi Sheril,
I would start by checking if there are actually dup key rows in both the target and/or the source system.
Something like this should help you find out:
select MANDT,VBELN, count(*) from R3<sid>DATA/VBUK group by MANDT,VBELN order by 3 desc
You should be able to determine if the problem really exists in the target library and if it is already an issue at the source system.
If no dup keys exist in the source table, you should try to find out how the source and target table contents differ from each other and if the problem is that the data has change (ie truncation) or if there are row that were imported multiple times.
Regards,
Luis Gonzalez