Managing a single blog and dedicating all your efforts to it is a good thing, but not when you have other ideas outside your immediate niche. You may run a blog on health because you went to medical school and be successful with that blog, but you watch football a lot, and you know a lot about that game. Don’t let your profession limit you to what you can write about. Create a new blog on sports or football and start off – you may even find football more interesting than the health you’ve been writing about.
There are people who are into automobile blogging, and have really been following celebrity gossip. Did anyone place a limit on your reach? Get into other things you can blog about, but create a different blog if the niche is entirely different.
There are people who in a bid to increase their income from blogging create different other blogs in the same niche containing the same exact content with their first blog. The only implication is that you may end up dividing the audience you already have and even reduce how search engine ranks your blogs. What you can do is hire someone to take care of the other blogs, so the contents can vary.