Hey All,
Have you ever come across this error message and thought what to do ? I certainly did 
During data loading to info objects,
DTP fails with ABAP dump "ITAB_DUPLICATE_KEY" with a short text "A row with the same key already exists"
This happens when a BW client has many different source systems, which gets added and removed now and then,so unused objects still remains, creating a duplicate entry in RSISOSMAP table.
Identify Unused Objects,
Goto SE38
RSAR_RSISOSMAP_REPAIR
Get the Details from here
Goto SE16
RSISOSMAP
Delete the Unused Objects.
Other reasons might be due to ACR has not run before next update,ensure aggregates are maintained,
Goto SE38
RSDDS_AGGREGATES_MAINTAIN
Give that info object name and execute.re load it should work.
All your comments are most welcomed and it's considered valuable. Correct me If I'm wrong ..