vtkSMPMergePoints::Merge is not thread-safe
By replacing the following two lines in vtkSMPMergePoints.cxx, currently in master starting at line 141:
vtkIdType firstId = this->AtomicInsertionId;
this->AtomicInsertionId += numberOfInsertions;
with
vtkIdType firstId = (this->AtomicInsertionId += numberOfInsertions);
firstId -= numberOfInsertions;
the problem should be solved.