3.2.4.0.1 The Problem:

The MPI-1.1/ standard says, in the discussion of MPI_INTERCOMM_CREATE, both that
The groups must be disjoint
and that
The leaders may be the same process.
To further muddy the waters, the reason given for ``The groups must be disjoint'' is based on concerns about the implementation of MPI_INTERCOMM_CREATE that are not applicable for the case where the leaders are the same process.

MPI-Standard for MARMOT