[masraum]

I assume the ideal would be that you scan a number into a column, and the spreadsheet tells you right away if that column already exists so you delete it then and there and go on to the next widget?

these might be some other options
Determine if column contains duplicate value - ExcelBanter

VBA Tips & Tricks: Detecting duplicate values (Excel VBA)

Spreadsheet Page Excel Tips: Using Data Validation To Check For Repeated Values

Avoid duplicate data within a column? - Google Docs Help