In one of my sessions, I had this one duplicant who died. I built some memorials and one of the living dupes buried it. When I noticed it, the dead dupe was running around and completing tasks like any other dupe. And since the name that dupe had isn't one of the default and I didn't name the dupes after the initial crew, I don't think that is a new guy.
Have one of your dupes die and get buried in a Tasteful Memorial.