Excel 2007 : Return Unique values in Drop Down List

You can't easily do that, but you can use a helper column that extracts a unique list from that range separately, then you name that new range and refer to it instead in your Data Validation.

So in a new cell, say B2, enter formula:

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and copy down as far as you need

Then in a new column enter:

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and copy down as far as you want in order for new entries to populate...

Then change your Named Range, Month, formula to:

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where E2 is where you entered the extract formula above.