I want to flip both but may end up keeping the duplex if I am able to get a lender to work with.

Having the CPA determines everything.In the general business field, an MBA will will hold more weight than an MSA for sure.

They do minimal upkeep to older property as tenants know they have a good deal for the market and don't want to (rock the boat).Conversely if tenants are paying top market they want everything under the sun and then some done to the property and want the best amenities.I already mentioned if a down turn happens the top market properties can get hit as people want to move to cheaper living arrangements.

Some things to look out for are the fees. environmental fees, excessive weight fees, another trick of the industry is billing cycles. some will bill monthly so you are billed 12 times a year others may bill every 4 weeks so you get billed 13 times a year (52 weeks/4= 13 bill)based on my experience the big guys tend to play ball and kill on volume to shut out little guys. also these little guys rely on the big guys station to dump, so they charge a base fee and mark it up or put add on to make money.

Because of federal fair housing laws that require landlords to allow reasonable accommodations for tenants with disabilities, the following apply: Landlords may not prohibit a service animal or companion animal from living in the unit.Landlords may not charge the tenant extra "pet" rent or "pet" security deposit.Landlords may not apply other "pet policy" rules like breed or weight restrictions.

For the 18 months that I hired them for their services, they have charged me over $11,000 for maintenance, repairs, & upkeep of this home.

Then while replacing the main sewer line the city sewer grunts drove their heavy equipment across our freshly excavated front yards & the weight crushed all our sewer laterals to the main.

Do you feel it is better to just go it alone so you don't have to have the issues of a partnership or do the benefits out weight trying to do it all alone?

It's not because they are making 3% by sending a contract over to a title company/lawyer and are collapsing under the weight of their moneystacks. 

Sometimes if people do not like the evaluation then they can try and stiff the appraiser with paying for it.With an appraisal for a bank, construction loan,etc. the appraiser has standards to meet and can be held more accountable so those reports tend to hold more weight on accuracy although an appraisal is just an opinion and not an absolute.No legal advice given.