Issue with conditional formatting in excel

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Could you be a bit more precise and specific in your descriptions? I'll give you a few questions to start you off; there may be more.

• What's "a while" as in "after a while it stops working"?
• What's the nature of the data in the cells that may or may not be duplicate? Is it text? Numeric?
• How does that data get into the cells in the first place?
• What's the rule? How does it appear in the conditional formatting dialog box?
• Is this a matter of a lot of cells in a long column, some of which are (or appear to be) duplicates?
• Does the "failure to work" happen in all kinds of different conditions (i.e., are there some duplicates that still are highlighted, but not all)?

Each of these questions gets at things that are possible causes. That's why it's important to get a more complete description, just as a doctor would want a more complete descripton of the pain you are experiencing, when it happens, where, the intensity, etc., before attempting a diagnosis.