How to avoid duplicate entries in same column

A different problem than last post.
This is an estimating spreadsheet. I enter a pert number and a quanity. The spreedsheet will give the part name, multiply the price times the quanity and add the tax. All this works well. I would like to not be able to duplicate the part number in the part number column. I have been using this for awhile now but I realized today that I entered the same part twice (two different line) and that is what I want to avoid.
Any help please
Thanks

I figured this out but now the question is can this also check against the same column on two other worksheets? I allow 3 sheets for part numbers and pricing and do not want to duplicate ant pert number from any of the pages