Developing Duplicate Check In Macro?

I've been assigned to develop a workbook that autopopulates a different workbook but they wanted it so the second macro is never messed with and the information is only entered once. Here is the macro i developed

Code:
Sub Monthly()
Dim wbthis As Workbook
Dim wsthis As Worksheet, wsm As Worksheet

[Code]....

Now The area in red is where the macro writes into the form. My thing is that I want to set up a check so there isn't duplicated entries. This is how the check would work. First it would match up any excisiting entries with JobNo and then check to see if the JobDate was the same. If both are the same then it would check Waste or CutTime, either one would work. If they didn't match, then those cells would be overwritten. If nothing matched then it would put in the new entry.