Android :: Automatic Selection Of Checkboxes Inside Listview

I am building a list and the list contain for every contact a checkbox to chose which one to modify for example, the problem is that when the list became longer than the phone screen and the scrolling is active; when i select a check box , a second one is automatically selected in the bottom of the list.the problem is the automatic selection of the second checkbox; please let me know how can i fix it? below is the code i am using for getView method.