There are some general rules but there is also room for flexibility built into the tax code depending on your specific tax strategy.Is your duplex separately metered?

It depends on the zip code and the specific property.

So, you're not "wrong" in thinking it's complicated.The tax code plays heavily into how certain transactions are categorized and there is some flexibility regarding what we can capitalize versus deduct.

Be sure to check permit and code requirements.5.

Quote from @Jonathan Greene: The difference is the low bar of entry.

In areas where you can find something since the bar to entry is lower, it will likely have a lot of competition and therefore hard to stand out. 

Also do you buy in towns like bar harbor or a little further away?

It puts each expense in the appropriate IRS code and punches out year end summaries that I email to my tax team. 

You MAY be able to ensure nobody is living there and avoid the need to use a constable/sheriff by having the municipality's code enforcement office post a notice saying the place is "uninhabitable".

To determine if the expense should be classified as a capital improvement or a deductible repair, the context of the expense is very important.An example is, if an item of expenditure is part of a general plan of modernization, renovation or rehabilitation to equipment or other business property, it typically must be capitalized even though on its own it would be currently deductible.According to IRS Code, expenses that you must capitalize are those that:Materially increase valueSubstantially prolong useful life (including replacement of deteriorating assets)Adapt the property to a new or different useHowever, you are allowed to deduct expenses and fees for routine maintenance and repairs that help keep the property in efficient operating condition.