Simple test case:
- Create new workbook
- Format column A as "Text"
- Enter the following into cells A1 and A2:
1234567800000639 1234567800000630
- Select Column A, and in the "Conditional Formatting" submenu, select "Highlight Cells Rule" > "Duplicate Values".
Excel is highlighting these as duplicates, even though they are different.
I suspect Excel is treating them as numbers and it becomes a rounding error, but I have specifically set to Text format and visually see different values, so I'm not sure how to tell Excel to treat it as Text.