Check previous values in transaction against current value
I have a sheet that collects transaction via a form. Each new transaction is added to the top row so I can pull vlookup to populate various other sheets. I'm trying find a formula that will compare entered transactions against prior entry of the same item. Using example sheet shown, if the "Tag Start" value for "Box 1" is different than the "Tag End" value of the previous entry for "Box 1" , I want to return a "Not Valid" response in the "Check" column, otherwise show a "Valid".
I don't even know where to begin with this one.