Runtime Error :ITAB_DUPLICATE_KEY Select on BSIS and BSAS Table

Hi Experts,

We get a frequent dump on the below select in Production system  for dupliacte key

select data from bsis into itab where bukrs on selection screen

                                                     gjhar  on selection screen

->dump on this select

   select data from bsas appending table itab where bukrs on selection screen

                                                                           gjhar  on selection screen

My ITAB is sorted table  WITH UNIQUE KEY  bukrs gjahr belnr buzei

I cannot replicate the same error in production and i have checked the database table and i am quite sure  that BSIS and BSAS cannot have duplicate

records for the key bukrs gjahr belnr buzei

I can make the itab non unqiue key and it will not cause this issue but i wanted to know if this scenario can happen?

Thanks