Can I restrict a random mob spawn to only allow one of that mob at a time?

I've created a boss-like mob for my Nether, and my intent is for him to randomly show up strolling around his domain. The goal is to fight him in the natural terrain, and it's an arduous battle. However, sometimes I end up with a second of the same mob spawning while I'm fighting the first one! Two boss bars, two attacking at once, it's impossible to defeat them both.

How would I best control the spawn to ensure that there is only ever one of these alive at any given time?