page cannot be refreshed without resending the information.

Hi

  I am working on a .NET 3.5 web application. I am showing a  modal window on click of  button in main Page.

I have a cancel button in the modal window.  I get the following error wheni click the Cancel button sometimes.

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Microsoft Internet Explorer
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The page cannot be refreshed without resending the information.
Click Retry to send the information again,
or click Cancel to return to the page that you were trying to view.
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Retry   Cancel  
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I get this error sometimes and when i click the button again it works fine. 

I am calling javascript code  window.close() on click of cancel button.

How to avoid this message?
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