Joe
Here is some good stuff from Debra.
From Debra Dalgleish
If your order numbers are in column A, with a heading in row 1:
1. Select cells A2:A50 (or to the end of your entry range)
2. Choose Data>Validation
3. Choose Allow:Custom
4. In the formula box, type a formula that counts the entries above,
that match the active cell:
=COUNTIF(A$1:A1,A2)=0
5. Click OK
Keep in mind that Data Validation is not foolproof -- users can copy and
paste invalid numbers into the cells.
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