There is not a lot of comps for duplex in the area but there one half duplex, almost identical to ours and fully renovated, sold for $75,000 two weeks ago.

I didn't want to start a bidding war so I sent identical assignment agreements to them all and said the first buyer to sign and return the agreement got the contract.

Presumably they aren't ALL committing tax fraud, meaning SALT cap must not apply to rental properties. 

I had something almost identical accept a smaller purchase price (and I don’t know how to calculate ARV ) and I had a friend that told me not to jump.

I've seen cases where an FHA underwriter goes "wait a minute, the borrower either lied about occupancy intent when they took out the HML, or they are lying to us now, either way this is a giant red flag for fraud." 

Property deed fraud is a criminal offense, so as long as the seller has net worth to lose, that should be a good discouragement too.

The seller is also selling me an identical lot, which I'm assigning to the same buyer for an end price of $52,000.

Our next property is almost identical to this (built at the same time by the same builder) and we plan to list for $800/month as a type of "rent experiment". 

Layout is identical to our unit.

You know this as well since you are from Memphis, but for those that may not - two identical houses in Memphis within 1 street of each other can achieve incredibly different results based on the street you are on.This is even more true for multi-family.