how many rows does the detail table have? if you have 4 header and 4 detail, then they will match 16 ways.
next, how are you getting the data? Using a stored proc/view? Joining to the tables directly in CR?
if joining, try to limit the number of columns and then click Database/Select Distinct Records. See if that helps. Selecting distinct will cut down on the number of rows, but it may not completely solve the problems, because if too many columns are different (that you are using) then duplication will persist.
HTH