This problem is summarized in this thread: https://groups.google.com/forum/#!topic/nipy-user/CPjld7rIsMk

Basically, running a workflow with MultiProc and asking for a specific n_proc runs (2 * n_proc) processes, with each job duplicated. I do not see similar behavior with the Linear or IPython plugins, and the behavior is occurring (for me) on an older CPU that does not do any hyperthreading fanciness. The double computation is definitely happening, as asking for a MultiProc execution with as many procs as I have cores ground my computer to a complete halt.

I'll try to put together a test workflow to produce this behavior.