Highlighting duplicate values - ERROR

[Johnson27]

I have a column of data in which I'd like to have the duplicates highlighted.
However, when I select this option from the Conditional Formatting drop-down,
it incorrectly highlights non-duplicates as well. What is the logic behind
identifying a "duplicate" and how can I prevent this error from occuring?

 

[HelpExcel.com]

Maybe they are not TRUE duplicates, i.e., padded spaces, etc..,