MVVM View Model Instance Maintainance problem in silverlight

Question

Hi, we are developing a project with silverlight using c# MVVM pattern. We are using a custom-control inside a page. The custom-control raising some events based on its dependency property change that is working fine when am using new ViewModel instance for the same page. But for business need we have to maintain the ViewModel instance for a page, at the same time we are not maintaining the page instance , every time the page instance created newly. Now If any property changed in particular view model whenever the particular page opened more than one time( closed then re-open) View model commands (events of the custom control) raising more than one time for single property change. I can understand that view or custom control's previous instance is remains somewhere and response for a corresponding view model's property change. what is the best way to resolve this problem.?