the night before last I dreamt that I told a DC that I existed in waking life and that she did, too. (why I felt sure, I don’t know.) she remained non-plussed. I wonder, has anyone unknown to you ever approached you in a dream and proclaimed that themselves a LD’er?
I have had that on some occasions 
And I always to tell them to go to LD4all then 
I’ve never had someone approach me and tell me that they are a LD-er, but like you, when talking to someone, and saying I’m dreaming, they would tell me they were dreaming as well.
she had no comment either way as to whether she existed in the waking world or not, too.
I was having a LD within a normal dream while dreaming (humor me):
In the dream I was with my dad and suddenly we knew that we had gone to sleep and we were dreaming. BUT “we” didn’t realize that we were actually in a dream within a dream (It’s a hard concept to grasp, I know, but the necessary words escape me at the moment). It went like this, we were together on a futuristic road, devoid of anyone else and we were arguing about a new model of a car when we both realized that we were dreaming. I was like “hey dad, we are dreaming” (but I failed to realize that it would be impossible to be talking to my actual father, who is also away on business in Salt Lake City most of the year 
). So I attempted to fly (my usual first action to prove that I’m lucid and establish my confidence in my abilities) but it wouldn’t work that well and I was straining to get up in the air. Then it was kinda weird, my dad just stopped moving and stared at me silently with a blank gaze. I ceased my efforts and the dream fell apart and I woke up.
Very strange, it was like 3 dreams in one… 