Please Note: This article is written for users of the following Microsoft Excel versions: 97, 2000, 2002, and 2003. If you are using a later version (Excel 2007 or later), this tip may not work for you. For a version of this tip written specifically for later versions of Excel, click here: Ensuring Unique Values in a Column.
When you are entering information in a worksheet, you may want to ensure that the data being entered is actually unique for a particular column. For instance, if you are entering a series of invoice numbers in column D of a worksheet, you may want to ensure that you don't enter the same invoice number twice.
There are a couple of ways that you can approach such a problem. If you are always entering your new information at the bottom of a column, you can use Excel's data validation feature to make sure that whatever you are entering in the current cell does not match anything higher up in the column. Simply follow these steps:
Figure 1. The Data Validation dialog box.
Please Note:
This article is written for users of the following Microsoft Excel versions: 97, 2000, 2002, and 2003. If you are using a later version (Excel 2007 or later), this tip may not work for you. For a version of this tip written specifically for later versions of Excel, click here: 
=MATCH(D1,$D:$D,0)=ROW(D1)
If you may be entering data anyplace within column D, and you want to know if you are entering a value that is elsewhere in the column (above or below the current cell), then you can follow the above steps, but use this formula in step 4:
=COUNTIF($D:$D,D1)<2
ExcelTips is your source for cost-effective Microsoft Excel training. This tip (2650) applies to Microsoft Excel 97, 2000, 2002, and 2003. You can find a version of this tip for the ribbon interface of Excel (Excel 2007 and later) here: Ensuring Unique Values in a Column.
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