I have a duplex I own free and clear, and just need some additional money to finish rehab.

How do I handle the rehab with renters in the house or is it advisable to let their lease end and have them vacate before I take on the rehab (since the additional bedroom and bath would be for a new tenant anyway? 

The additional capital helps too :)

There are often hidden costs--such as: engineer certification of the structure (depending on what the city requires) smoke damage to portions of the unit(s) that are "not affected" additional insurance during the rebuild, sealing the framing that has smoke, etc. 

Who would have guessed Wells Fargo takes 2 additional business days to wire out after charging $30?

Oliver is not currently a great  neighborhood but I believe with the development the city and Johns Hopkins is pushing in the area make it a good long term bet.

In addition, learn everything you can:1) Read books2) Listen to podcasts3) Go to real estate meetups4) Find ways to add value to a potential mentor.

As your assets increase you may want to get a personal umbrella policy (PUP) to cover any additional liability.

If it's as close to downtown as you say, I would bet that they are already owned and not on the market.

On the inside, think about every doctor's office you've ever been in: the major additional system comcern is that Plumbing is everywhere.