Is there any way to have one and only one instance of each activity?

in Manifest Activity property you can give this parameter android:launchMode="singleInstance"

Read in more detail here http://developer.android.com/guide/topics/manifest/activity-element.html

This is your flag! http://developer.android.com/reference/android/content/Intent.html#FLAG_ACTIVITY_REORDER_TO_FRONT

intent.addFlags(Intent.FLAG_ACTIVITY_REORDER_TO_FRONT)

Note the 'addFlags'. Also note that, onCreate will not be called on this Activity when a new Intent is delivered to it. It will be delivered via the onNewIntent().

This does not ensure that there is a single instance of the Activity running. To ensure that, check this out: http://developer.android.com/guide/topics/manifest/activity-element.html#lmode