Duplicated relationships when copying activities?

Has anyone copied a set of activities and had the logic duplicate, namely if there are SS/FF relationships?  'Copy relationships only between copied activities' is selected on the copy options.  My P6 (v 6.0)  has added a duplicate relationship, creating a FF where there's a SS; start/finish dates are not affected and both show the same dates.   I've had to delete all these bogus relationships (very time consuming!).  It's happened more than once, but doesn't happen always.