assuming you want just the last of equal groups of contact_objid and account_id, you could use by group processing (even if you need to add the notsorted option)
[pre]data new_data ;
do until( last.contact_objid ) ;
set original ;
by account_id notsorted contact_objid notsorted ;
end ;
run ;[/pre]However, this method depends on having actual Account_id rather than NULL.
good luck
PeterC