When a model notices that someone is imitating him and takes the imitator as his own model in return. The more that each party is drawn into mimesis and doubles down on the rivalry, the stronger the mimetic bond between them becomes. Each side is bound to the other (the double bind) because each side takes the other as a model. You could think of a mimetic double bind like two atoms or molecules that are bound together because of the way their energies are oriented.[1]
[1] The term double bind comes from the work of anthropologist Gregory Bateson, known for his theory that schizophrenia originates in dual and contradictory messages.