|Please understand that this is not a push-back; I'm just trying to understand what you found different between the 2 sites. |
An explanation may help me word my future responses to questions in a way that will help more people.
Both Chip Pearson and the author at Mr. Excel use the Data Validation feature to present a message if duplicate data was entered in a given range. They both use a formula that evaluates to either True or False and they both present the message if the formula evaluates to False.
The only difference is the actual formula used.
If we want to take this a level deeper, one could argue that Chip Pearson's method is actually the better way since Excel only has to evaluate a single function, not two. (Note: I have nothing against Mr. Excel and have sought and used information at that site many times.)
Once Mr. Pearson's function evaluates the COUNTIF function, it's done. Mr Excel's method requires Excel to first evaluate the VLOOKUP function and then evaluate the ISNA function.
While this inefficiency may be minor in this singular case, a multitude of these types of functions could impact the performance of a larger workbook.
All that said, I'd be interested to hear why the method used at the Mr. Excel site was easier for you to follow.