It's often the case that duplicate Recordings exist that are each linked to the same Work, but with different attributes (e.g., one is a "recording of" whereas the other is a "cover recording of").  When these Recordings are merged, there will be 2 separate work-recording performance relationships because of the different attributes (cover, partial, instrumental, medley), requiring someone to manually delete the duplicate copy.  Most of the time, merging the attributes into a single work-recording performance relationship will be preferred, as it is safe to assume that the entry with fewer relationship attributes is incomplete data, or data that predates the existence of the relationship attribute.