A bivy is not required with the Duomid. It is very much full coverage, however, there has been one time where I was in a substantial rain storm and the drops would hit the floor beside the Duomid and splash under the edge onto my bag. Nothing that a decent bag couldn't handle but worth mentioning in case you camp in torrential rain on a regular basis. I would think that almost no one uses a bivy in the Duomid but others can chime in if they disagree.
I haven't used mine during bug season, but I find it tough to get a pitch right to the ground all the way around. Even if I put the pegs right through the loops on the corners, the front and back have a small gap in order to get the pitch tight. I think if you are dealing with a moderate amount of bugs, you may want to select a different shelter, ask Ron to add a bug net around the perimeter of the Duomid, or bring additional bug protection for inside the Mid.