title: Prevent dialog to be opened multiple times when resuming activity
body: <p>In my Android application, in order to ask the user if he/she wants to resume a current game, I display a dialog saying "Do you want to resume current game? Yes - No" on the main game activity.</p>
  
  <p>The thing is that if I resume various times this activity without answering the dialog, then I get several dialogs, on top of each other, which is obviously not my goal.</p>
  
  <p>I could easily avoid this behavior using a Boolean var, but I was wondering if the Dialog class had a kind of option preventing to be duplicated or something of the kind.</p>