Buying a Duplex Property

[Prachi Metha]

I am looking into buying a duplex property in Sacramento region. I have seen few on the market that stay in market for a longtime or get back in market every few days. All these properties seems to have tenants as well so I am not sure why these properties stay in market for longtime.  In general what should be few points or notes I should look into while buying such property.  Is there any major difference while investing into single family vs duplex property? Please advise