i did`nt get it !!
for e.g :: if i have 1btc can i double spend ?? means i can use it two times in two trx`s?
but i think if i send 1 btc to someone And the someone send the 1 btc to any one then 1btc will be double spended

Theoretically yes, but only one transaction gets a confirmation and the other one will be detected as a double-spend.