The basis of the original property is divided appropriately and depreciation is allocated prorata so that your adjusted cost basis is accurate and you then have only the allocated gain from that one unit to content with.  

You said it was a duplex and you;re in half so you have to allocate the insurance, the taxes, and the mortgage interest because the personal part of your interest and taxes is reported elsewhere.

If you're in the wealth building phase, it's a no-brainer to arbitrage the extra money and allocate that towards an asset with a higher return than the mortgage paydown. 

Great options if you are looking for full-blown property management options.If you have multiple rentals you'll need the version of Quickbooks Online with "classes" ($85 per month to @Simon W. point) to allocate the income and expenses between different properties and their chart of accounts (list of income and expense types) doesn't come setup out of the box for real estate.For real estate-focused "accounting & tax software" I'd recommend Stessa (Free to start) and Digb.com ($1 a month to start) and Quicken is also a cheap option like @Bill Brandt mention.

If you reinvested the cash returns into an additional deal every 3-5 years, while also allocating even more additional personal capital each year, the compounded cash flow 10-20 years down the road, once you now have 5,10,15 different investments, looks very solid.

It was just somewhere along the way that I decided to allocate those (mininal) funds to something else.I go through phases of super frugality where I cut any extra expenses....BP did not make the cut lol

Your math is off.You spent $5200 to bring the mtg current.You haven't included any transaction costs ... for 2 transactions.You haven't allocated a single dollar to the displaced homeowner.

You can also allocate your proceeds in any manner. 

If you’re willing to allocate the time and are competitive, Cleveland’s lower end market may be a suitable fit for you.

If you live on it, and rent out the room, the repairs will be deductible that is allocated to rented portion of the house.