Data Validation Example
The Data Validation tool is a great tool that allows you to control what gets put into a spreadsheet.
In the Requisitions column, create a Data Validation so that the same number cannot be used twice.
You can either create a separate column (D) and do a check using countif, then use data validation in Column A to check against column A.
Otherwise, build the check directly into the Data Validation (using the Custom format).
Now try and add some duplicate transaction numbers
Refer to the adjacent solution sheet in the spreadsheet
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