Controls whether this SDC entity prevents a new adjacency from forming to a neighbor presenting the same System ID but a different SNPA address as an already-existing neighbor on the same circuit and level, defaulting to true; this can be changed while SDC is active, though doing so only affects whether further adjacencies form, not existing ones. An admin disables this only in unusual scenarios where duplicate-System-ID neighbors are expected and intentionally tolerated, which is atypical.