Plastic SCM implements full merge tracking. Have you seen the green arrows between revisions on the 3D version tree? These arrows are not just decorators, they are used by Plastic to keep track of what has been already merged.

If you work on a branch, merge your changes to another one, continue working on your branch and merge back again... Plastic will calculate what exactly needs to be merged. You don't have to remember which were the revisions already merged, Plastic will take care...

The screencasts shows a rather complicated scenario: two developers in parallel, an initial conflictive merge and later on a second one that gets automated thanks to the Plastic SCM repeated merge support.