i have a datastore

i reload it using :
(like when i want to add new record to existing grid thru a control in grid toolbar)

after this when i click on column header for remote sorting
my grid is showing double the records existing why because

i have set add:true
and the new set of sorted records are appending to the set of unsorted records
i want the existing records to be replaced by new set of records cached
how can this be done without using reload function's add:false

please help me
thank you