I do recall when this was possible, but to the best of my recollection, it was the update from 10 to version 11 where we saw this changed.
You can use the Hide Object when property new to FileMaker 13 to hide the 2nd ... nth instances of an object in the body if you can set up an expression to identify when a given value is the first in the sub summary group.
Say your sub summary sorts on a field named "category".
your hide object when expression might then be:
YourTable::category = GetNthRecord ( YourTable::category ; Get ( RecordNumber ) - 1 )
This compares the value of category for the current record against the value of category in the preceding record and hides the layout object if they are the same.
Thanks, the hide object using GetNthRecord did the trick.