Drop Down list with unique values, without blanks and without making a hidden column

Date of Birth 20/01/2016 20/01/2016 10/05/2014 15/06/2012 08/06/1999

I need to create a Drop down list using dates of birth like those shown above.
It needs to ignore blanks and remove duplicates to leave only unique values.
Is it possible to do this without adding another row?

The data will be extracted onto excel.
It will then conceivably provide up to 20 dates of birth.
The users are not excel literate, copy paste is the extent of their ability as civil servants.

This is why I would like to provide a list for them based on the extracted data without hiding any cells as they will find a way to break it.

I have provided a spreadsheet with the same details within this post for your convenience.

Is this isn't possible, please let me know. Thanks.