In this article, we present two methods to de-embed the filters' responses from a diplexer whose junction is known. A weak identification problem is here described by means of a dual-state junction and can be applied for general diagnosis of prototype diplexers. A strong identification problem based on rationality of the filters is also introduced here and is suited for a real-time tuning process. This technique is based on a polynomial approximation derived analytically from the algebraic structure of the diplexer's response. Additional unknown delays violating the rational assumption can be moreover detected and estimated.