I was write the following code in form closing event .The form ask me two time Do you realy want ot close ? How can solve this problem ?

Don't use the Application.Exit call - all you need is the default e.Cancel = false and the closing of the main form will close the whole application automatically.

When you call Application.Exit, it tries to close the form again - so you get the query again when the Closing event is re-called.

Some additional information to existing answers:

First of all, if something happens twice instead of once (a pretty typical problem), the first step to dig out the reason is using the debugger's "Call stack" window. Put a break point inside the called method, say, in the first like of the implementation, wait until it is called first and second time, in both cases, look at the call stack, to see where the call comes from. This is easy.

Also, when handling FormClosing (or, even better, overriding OnFormClosing) in such situations as yours, check up the reason:
https://msdn.microsoft.com/en-us/library/system.windows.forms.formclosingeventargs.closereason%28v=vs.110%29.aspx[^],
https://msdn.microsoft.com/en-us/library/system.windows.forms.closereason%28v=vs.110%29.aspx[^].

You only need to cancel closing of the reason is System.Windows.Forms.CloseReason.UserClosing. In all other cases, you don't need to do anything else, and should not cancel the event.

class MyForm {
   // ...
   protected override void OnFormClosing(FormClosingEventArgs e) {
      if (e.CloseReason != CloseReason.UserClosing) return; // will be normally closed
      // now you can check up if something needs saving
      // and not show your dialog,
      // depending on result, set e.Cancel to true...
   }
}

—SA