He has had a few people out with bids from 4500$ to add a slope and shingle to 1200$ torch down bitumen.

(from a Roofer) For a residence (duplex), it's probably best/easiest to go with an asphalt type material (as Eugene mentioned), like Mod Bit (Modified Bitumen). 

If in a residential neighborhood, and most of the houses have shingles... your flat roof may be just fine with using Mod Bit materials (Modified Bitumen).  

Out of pocket, that's just too high- The roof is modified bitumen and is coated with aluminum oxide.

The building tradesmen are busy too, but I just called ABC and got the name of a guy who buys alot of bitumen roofing so as to get a quote on a leaky patio cover, got a guy to bid cement board skirting on a stick house by calling Palm Harbor mfgr home sales center, got a card from a guy working across the street doing drywall repair and put it in my phone.

It gives me planning certainty and fewer maintenance calls.Average roof in my area is around $5-6k, with 3-tab asphalt shingles, bitumen underlayment, and new plywood over the existing sheathing.  

Are you saying its possible that the roofer stripped off the old bitumen/tar roof and found rotten sheathing but instead of replacing it they just laid down new felt over it and torched it down?

Roof is bitumen. 

@Courtney Rollins I just put a new bitumen roof and gutters on a rowhome I own in the city and it cost me 6k all together. 

any flat roofers know best way seal a leak in modified bitumen roof when cant find starting point just end point?.