this would be my approach:
global stringvar displayedItems;
if instr(displayedItems, {table.field}) = 0 then (
displayedItems := displayedItems + "|" + {table.field} ;
{table.field}
)
else
"duplicate"
There are lots of things that can be done, but not knowing too much about the database and the exact requirements of the report, this is an 'outline' to demonstrate the functionality.
This would be placed in a formula and the formula would most likely be placed in the Detail Section of the report.
HTH