My GC says a duplex cost more than two SFH's?

[George Lods]

Hi everyone! I'm finally about to get started and decided to go the "build to rent" route. That said, the GC I am looking to use has told me it would cost me more to build a duplex vs two sfh's (same sqft) r/t firewalls etc when it comes to additional steps for adjoined units. I was thinking it may be closer to a "wash" or, maybe only slightly more only bc w/ a duplex at least we aren't paying for two exterior wall materials for two sfh? I'm asking here for anyone who has knowledge bc my GC has really only built SFH, never any MFH. So I'm wondering if he's just not too familiar. Other than him not having built MFH before, I like everything else about the GC. Any first hand experiences that can be shared would be greatly appreciated. I'm not grossly opposed to SFH if it is much cheaper, but do to the spacing between dwellings per city code I could fit more doors on this plot if building duplex. Thanks in advance for any info you all can provide!