Duplex are better than single family in terms of unit diversification in addition to more cash flow.

Or maybe the stock market with a decent yield on investment (diversification!).

Jobs and diversification of employment are very important. 

There is no industry or diversification of employment to drawn stable tenants, as well as to bolster the prices of houses. 

@Nicholas Patrick Real estate is a great investment to use for diversification purposes.

You might also want to consider passive approaches to learn or a combination (hybrid investing - some active locally, some passive outside of your geographic reach for diversification).  

Look at economic indicators such as employment, basic supply and demand, and diversification of employment.

If you like to be somewhat active, great, do that locally but for distance and diversification, it's worth looking at being a part of institutional grade $50-125M projects in strong markets that offer potential cash flow, tax shelter and appreciation.

Overall I like the diversification. 

This extends way beyond investment advise, but important and true none the less.As for diversification, please do not confuse an active investment from a passive one.