Merge bug

Hopefully found the ultimate cause of duplicated (completed) tasks.

The corresponding test case

 1. does not rely on the fact that there are modifications with equal
    timestamps and
 2. is a LOT simpler than the other ones.

src/TDB2.cpp

@@ -867,9 +867,27 @@ void TDB2::merge (const std::string& mergeFile)
       std::list<Taskmod>::iterator current = rmod_it++;
       Taskmod tmod = *current;
 
-      // new uuid?
-      if (tmod.isNew ())
+      if (uuid_left.find (tmod.getUuid ()) != uuid_left.end ())
       {
+        // check whether the remote side has added a task with the same UUID
+        //  this happens if it inserted a modification with an older timestamp
+        //  into the undo.data and thereby moved the branch point to an earlier
+        //  point in time. Normally this case will be solved by the merge logic,
+        //  BUT if the UUID is considered new the merge logic will be skipped.
+        //
+        //  This flaw resulted in a couple of duplication issues and bloated 
+        //  undo files (e.g. #1104).
+        //
+        //  This is just a "hack" which discards all the modifications of the
+        //  remote side to UUIDs that are considered new by both sides.
+        //  There may be more issues with the algorithm; probably a redesign
+        //  and proper encapsulation of the merge algorithm is due.
+
+        rmods.erase(current);
+      }
+      else if (tmod.isNew ())
+      {
+        // new uuid?
 /*
         // TODO Don't forget L10N.
         std::cout << "Adding new remote task             "