multiple times event calling on button click.

Question

I have a button on in silverlight page. And i m calling WCF method GetData() on button click event
for return result i have attach the completed event below the method call
When i click on button first time then it works perfect and i am able to get result in completed event.
But when i click again on a button then that methods call one time but i am getting the result twice...
Means, mulitiple event calling is happening on a click of button.....So how i can fixed this .........Because I am performing some operatin in the completed event and because of multiple calling its fails the logic inside the event.