Do you use an app, do you buy your lists, or do you hand write and do it all yourself, if you have used two different methods which do you prefer? 

I think Sac has "earned" its property value increase with an increase to the economy from big companies like Oracle, Apple, the MLS, etc. coming into the area, and the continued growth of new development and homes to be lived in by those corporate and state workers.

Is there a reliable phone app for our tenants that anyone has experience with? 

You can use apps to schedule posts.

In one of Mindy Jensen's podcast she mentioned checking rent is free if you download the rentometer app.

My contact info should appear down below in my signature block, provided you're not using the phone app.

I sent 2 requests for payment through cash app (that’s how they sent payment before) but no response.

Depending on the bigger picture of this investment I'd recommend that one of you (perhaps actually both of you) read Brandon Turner's book about getting started in Real Estate investing:https://www.biggerpockets.com/...Duplexes are basically single family houses, except less desirable when compared apples to apples, so all of the basic info about general real estate investing applies to duplexes, triplexes, and 4-plexes with that important provision. 

I like to keep track of my tasks using an app called Todoist.

Apples to apples.