@Susan Hill
If you're really making $7,000 a month in profit from a single duplex, after the loan payments, vacancies, all maintenance & repairs, cap ex, utilities, landscaping and all other expenses - I don't see why you would sell. I haven't seen those kinds of returns anywhere. I typically profit about $250-350 per unit on my deals.
With that being said the question of holding vs selling always comes down to - do you have a better place you can put your money (return wise)? If the answer is yes, it makes sense to sell. If no - then keep it.
@Christopher Grobbel
As far as finding PMs - see my other answers, basically always find one through referrals.
As far as keeping PMs accountable - develop processes and strategies for how you want thing done. For example, leasing guidelines, make-ready scope of works and pre-approved budgets, procedures for dealing with late payments and evictions, etc. Get it all in writing, send this to your PMs and go over to make sure you're on the same page.
I also like to do a phone call at least once a month with each of my PMs to check up on things, answer any questions they have, help them with any problems, etc. And then obviously check all of your statements, leases and deposits.