What concerns should I be aware of that are common with duplexes?

One concern I would have is the durability of those rents.

Anyone else concerned with the debt crisis?   

Im less concerned with actual areas and just more interested in trying to figure out the best size of the property as I always hire property managers.

My concern is you're going to get yourself tied into a contract and be financially liable for something that you're not ready for.

I just saw a mobile home park come available in my city of Louisville, Ky by owner.  

For me personally, I started with single-family homes then went into mobile home investing buying properties for all cash.

A house or a mobile home on a main road, not held completely to the sfr zoning as development moves into an area main road frontage can be turned to commercial all the while you are renting it as an sfr until that potential shows up.For example, we bought as a tax deed 2.56 acres on Normandy blvd in Jacksonville Florida with 2 houses and a Mobile home for 49k, but it also has 528 ft of frontage on Normandy blvd  which is SR 228 & 330 ft on the side rd.

My portfolio has performed beautifully for me in the past but I’m concerned about a future SF earthquake and wondering about the opportunity cost of staying in San Francisco relative to investing elsewhere.

My primary question I seek to have answered by posting this post is fairly straight forward, yet I encourage debate concerning metrics and comparisons.