How did you define the field in the Schema Designer? (I assume as an SREL to the cnt object). An SREL is only used to define a one-to-may (in this case, one contact to many incidents) relationship. If you need to define a many-to-many relationship (an LREL) then you need to create another object (table) that contains two one-to-many relationships.
Search the community for examples of creating custom LRELs.