Don't add markers to the selection multiple times when adding a list.

git-svn-id: svn://localhost/ardour2/branches/3.0@11357 d708f5d6-7413-0410-9779-e7cbd77b26cf

gtk2_ardour/selection.cc

@@ -1045,6 +1045,9 @@ void
 Selection::add (const list<Marker*>& m)
 {
 	markers.insert (markers.end(), m.begin(), m.end());
+	markers.sort ();
+	markers.unique ();
+	
 	MarkersChanged ();
 }