Find doubles or more of number in one Column and mark them

Ahoj Martin,

Do you really need a macro for that?
It can be done easily just with conditional formatting. I selected range G2:G419 (whole column G in the formatted table) and used such formating rule (based on a formula):

Formula:

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See in the file - the formatting of cells changes automatically when value in cell C8 changes.

In Main Tools you can find Conditional Formatting icon, and there is Manage Rules, where you can see ruls used and edit them as needed. See first few seconds of https://support.office.com/en-us/art...4-1883e40848f9 if you cannot find it.

Then note that the conditional formatting I used in above solution is a rule based on a formula (see for example: https://support.office.com/en-us/art...e-1da7880c750f and don't worry it's Mac version - it works the same on Windows platform)