Hi,
Here is a link to a file that answers Chandoo's question.
vacation-calculations.xlsx
The formula in cell G19 is an example of how, given a data set with non-unique Identifiers, how to count the number of unique elements meeting a certain criteria.
In this example, we have a list of employee names and the dates when they took vacation and we want to know how many unique people took a vacation between two given dates.
Hope this helps.
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