title: Prevent multiple instances of my Android application composed of a single activity
body: <p>I have an Android application which is composed from a single <code>Activity</code>.
  How can I assure that only one instance of my application (== <code>Activity</code>) will exist in a given time?
  I got to a situation in which I succeeded to open multiple instances of my app by clicking on the app icon multiple times (this doesn't reproduce all the time).</p>