You need that additional "instance"--what is more commonly called a Table Occurrence and that's the name that appears in FileMaker 13 when this happens.
FileMaker frequently needs to evaluate entire chains of relationships in order to determine what data to access for display on a layout or in order to execute a particular script step. Thus it can't have 2 different "paths" through the same set of table occurrence boxes. That added "instance" allows you to link to a table that would otherwise create a "loop" in your relationship graph with two different ways to evaluate the same set of relationships. (But you often have to say "no" to this message as FileMaker may have guessed wrong as to which of two tables needs the new occurrence.)
Some suggested resources for dealing with this issue: