Of course it's not overkill. Tail lights are the most over-analyzed subject on these forums. If you ask a question about tail lights it brings out dozens of amateur psychologists claiming certain configurations of blinking/solid lighting will draw drunk drivers toward you. Other "Pretend PHds" claim the opposite. We are all experts, and we all have "studies" in our back pocket ready to whip out. We all pretend to be Forester, or at the least Robert Hursts or (worse or better?) Grant Petersons.

Put as many lights back there as you want. The brighter the better. If you want them all blinking, then blink 'em. Experiment. Find out what makes cars steer clear and what doesn't. It's your bike, your city streets. Screw the psychoanalyzing, full speed ahead!!