Received a 1099-K for rental income from a payment site and received a 1099-MISC from rental property manager...how do I account for this, and not have duplicate income?

They are not required to report the income to you on both documents. Do not report double the income.  These situations have become confusing for the agents issuing the required tax documents. Until the IRS cleans up the rules follow the advice below.

Keep good clean records to show you only received the money once.  Report only the income you actually received. 

Keep those records with your tax return to show the actual income received.

It's not necessary to add income and then remove it as an expense.  Your records will support your tax return if and when you should need them.