I really don't think that the city or county will care. In MN zoning ordinances normally read that a property may contain a less dense use but not a more dense use. In other words a property that is zoned to 2-Family residential can normally have a single family residence - but not the other way around. That said you will need to check with the city in which the property is located first.
Architecturally you may have some more headaches. Many duplexes are set up such that a party wall is between them and in some cases this wall is made out of concrete block. There are too many variables at play to be able to give you a complete answer here. My recommendation would be to talk to a contractor once you have a property selected and hire a residential designer to come up with some ideas for you. I would be happy to recommend both for you if you like.