I've had this happen a few times, but having a tank/healer fill the last spot in DF queue and then immediately quit, or wait for the timer to nearly end before withdrawing repeatedly is absurd. This morning I went through about 30 minutes of (what I assume is) the same tank joining and withdrawing from ST queue. I assume it was the same person because I had been queued for a while before the duty finally popped - my wife was separately queued as a healer - and after 20 minutes of waiting we were in the same queue.

My suggestion would be to apply a 1 minute restriction to joining DF queues if someone chooses to withdraw. This prevents a tank/healer/dps from repeatedly filling the same spot and wasting everyone else's time (and hopefully would let a serious player enter the queue in the mean time). At the same time, if someone withdrew from queue because they are finishing a fate/whatever - a 1 minute penalty shouldn't really affect them.