Hi Rodney,
looks like you came to the right place for advice - there are several really good answers here to ponder over. My first investment was a duplex - though I lived in one side of it. Eventually I moved and rented out both sides.
Some of the advantages I found in my experience was the ability to cover the bulk of the mortgage with only one unit. If one tenant did not pay the rent, then at least I had another rent check coming to help cover the mortgage. This, to me, was one of the greatest advantages - I felt secure that I would be able to cover the mortgage payment each month. With a SFR, you don't have much wiggle room for slow or no payments. If your tenant does not pay, you must cover the entire mortgage on your own now - If your evecting, this may continue for several months - with a duplex, at least you would have another rent check to fall back on.
Also, when you consider taxes and insurance - it would cost more in taxes if you purchased two homes and paid for two insurance polices Vs paying taxes on one property and only one policy. So you cover those things under one roof. Cost of acquisition is doubled also since you would be have to buy two houses. ( I know you are just comparing a SFR to a duplex - not two houses, but assuming you plan to acquire more )
This is why I have a personal preference towards multi family units over SFRs. That doesn't mean there aren't some great deals out there with SFRs - at the right price and location, there are several great investment opportunities with them - but given the choice, side by side, I would rather buy and hold a multiplex - which in my area are actually in high demand now.
Some obvious disadvantages are if both tenants stop paying rent, your going to have a big nut to crack - and as others have mentioned - multiplexes usually have the water lines tied to one account / meter and the owner is typically responsible for water payment ( unless negotiated in the contract). Also because usually all uits are the same age, you have the same thing go wrong around the same time, costing double for repairs -
Ex: one 20 yr old AC unit goes bad - two months later, the second 20 yr AC unit goes out.
so those are things to think about...
As a first time landlord 3 tenants may seem a little intimidating at first, but you may find that much like wild animals, many times they are more scared of you, than you are of them - just don't let them smell blood, cause they can still be dangerous. But, with a little practice, plenty of communication, and proactive landlording - you'll be looking for as many tenants as you can get- Good luck with your investments - always re-run the numbers and account for the unexpected ( like replacing a HVAC unit..or two) Keep us updated with what you decide. Best of luck to you.