Several times a day (and sometimes at night, too) my mom asks me who is living here. I explain that it's her, me and my father (although my father is in a rehab facility temporarily right now). She insists that there is another person living here, and always questions my response that there isn't. My mother is 90 and displays many of the symptoms of dementia, although she hasn't been officially diagnosed with it (she passed her last memory test, but her memory has been on a sharp decline since then). It doesn't bother me, really, I'm getting used to it - it, like so many other things, has become the "new normal." But I've never heard of this specific symptom before and wondered if its common.