A duplex should have a lower cost foundation and typically has less windows and exterior.

The comps however all have central AC, whereas this property doesn't (has window units). 

Also, properties on the lake cost 3-4x per SF and have a major downside for Dec 1 - April 1 or so: they see a giant mud bowl out the window because the lake is drained.

We decided to replace wooden windows in a 2300sf flip we were doing, instead of rebuild them on a 1908 house.   

I'm running into hangups with code section R302.3.1 which seemingly requires not only the floor separating the units to be fire rated but all the walls below supporting that floor to be fire rated (ie all exterior walls and load bearing interior walls on the first floor), which may mean extremely expensive fire rated windows at the lower unit.  

How are flippers purchasing items such as Tile, LVP, Windows, Doors and so on at an extreme discount?

New windows, doors, Lp siding, fixtures, flooring, Sheetrock, kitchen, bathroom, trim and stairs.

If i decide yes, how can i lock myself down in addition to security systems, cameras, legal self defense, and anti-theft door/window stoppers?

We have a tenant that is smoking out of the window of a non smoking apartment and it is bothering other tenants.

If I don't hear from them (or if I intend to non-renew their lease) I send them a 30 day notice to quit when we're 30 days out from the lease termination date.I state in the lease that I'm allowed to post "For Rent" or "For Sale" signs on the property and / or post "For Rent" signs in the window of the unit which is to be vacated, and further state that the tenant agrees to showings of the property with a 24 hour notice.