I’d like to buy a duplex in Florida with a pool. Rent units out but perhaps build an accessory dwelling unit on site so that I could visit sometimes as a vacation. (I live in the northeast)

Thoughts?

You need to check the local zoning ordinance. I can't speak for Florida, but I would expect not. ADU are typically only permitted with Single Family Residence Lots. I would start with looking at the local zoning code

@Raymond L Tennal Jr

How about a single family?

Leave it vacant for when I’d want to visit?

That could work.  Alternatively, get someone to manage it as and Air BnB.   I'm thinking of doing this with my Place in Washington State. 

You need to understand the Florida ADU regulations. (And local City regulations). I don't know how it works there but in California you can put ADU's on Single and Multifamily zoned lots. In fact Multifamily zoned lots allow two detached ADU's.