I am actively looking for a duplex or fourplex. Cap rates and a lot of other metrics people will try to use on them belong to commercial (5+ unit) properties which are valued very differently; as a business rather than a structure. As Robin mentioned, make sure your cashflow is sufficient. I also look at the ratio of cashflow to purchase price (while also taking into consideration the overall property value). I found a $550k duplex earlier today that I would really like to have, but not at that price. Rent would be ~$1,850 per side, giving ~$3,700 in gross rent. By the time all is said and done, I'd realize, what, about $400 per month in net cashflow for my $550k purchase? Even though that property is all new and pretty, other duplexes in that area are selling for a $200k less yet commanding close to the same rent, albeit a few years older, and would therefore give a lot more bang for the buck.
That's also something you can use to establish a value for them. Most municipalities will zone particular areas for residential multifamily dwellings, so there are not too many lone du/tri/fourplexes out there. If the neighborhood or multifamily street does not often have a property come up for sale, that may be an indication of neighborhood stability, and depending on the greater area, value upswing potential. If you find one of those few lone multifamily properties with no others around it, you can use that to negotiate a lower purchase price. "Mr. Seller, there are no other properties around here justify your asking price. So I can only offer $xxx."