Duplex of SFH as first investment

I am currently in the beginning stages of buying my first rental property. I am trying to decide if I use most of my savings to buy a SFH and then start the saving process over again to buy a duplex which could take a few years or do I wait a while and save up a larger sum to buy a duplex first?

I am at the early stages of researching and learning so I am having a hard time understanding how investors acquire multiple properties a year. My wife and I make very good money as a travel nurses but it would still take us a while to continue to come up with these 20% down payments for each property. Any advice is greatly appreciated! 

I am in the OKC area but also open to investing out of state.