identify interface boundary nodes in parallel
 

I'm trying to find a way to identify whether a node is on an interface with another processor such that if I wanted to export a node list I could avoid duplicate nodes. I tried using BOUNDARY_NODE_P(n) and INTERFACE_NODE_P(n), however, the first gives all boundary nodes (including those not on the interface) and the second doesn't appear to ever be true.

I have also tried looping over the faces on each processor and determining whether it's a PRINCIPAL_FACE_P(f,t) and then marking the nodes to prevent them being counted twice, however there will still be duplicate nodes from the edges that are shared along the processor partition. Any insight or knowledge of an internal function to identify the interface nodes would be appreciated.