My Mapplex seems to be acting up. I have a road feature, and its label disappears or multiples as I pan around. I have remove duplicates on so I shouldn't get multiples and I don't pan away from the feature so I should always get a label. It also does this when I zoom in and out.
What version of ArcMap are you using? This could be to do with the data you are using, either there are multiple feature classes all the same or you're using multiple label classes on the same feature class. One of these would account for why you're seeing multiple labels (remove duplicates only works within a label class). As far as disappearing labels go, I'm not sure what could be causing this, is the label unplaced or just plain gone?