@Derek A.
-Should I verify the duplex tenants are paying their rent and how should I do this?
* Ask for a copy of the signed rental agreement. You could also request a copy of the tenant ledger so you can review their payment history.
-What will need to be done if purchasing the duplex with tenants who have a lease with the current owner?
Varies by state. Regardless tenant rights remain. Would not hurt for simplicity purposes to get the lease agreement in your name.
-What type of insurance do I need? Can I get one policy to cover all of my business dealings or do I need separate policies for each property?
If you are getting conventional financing your lender will require insurance against the collateral they are loaning on. Insurance "for all your business dealings" would be an umbrella policy and those never hurt to have.
-Can I rent out a property that I purchase on contract for deed?
Yes. Unless they put it into the agreement that only you are to occupy the property. So be clear with them of your intentions and make sure that is not in the agreement.
-How should I collect my rents?
With a baseball bat! :) j/k clarify your question. there are a lot of online services that will let your tenants pay online or at a convenience store and that online service will directect deposit into your account. You can use a property manager. You can have them mail you a check. Options are endless.... as long as it is in USD and the check clears not sure how you get it is as important :)