I have a column in which the cells are a pull-down list that already uses
data validation to get entries from a column on another worksheet (The values
entered are phone numbers.).

I would like to also check this column for duplicates upon entry (from the
pull-down list) so that the same phone number does not get assigned to two
people. It does not appear to be possible to use two different types of data
validation on the same set of cells.

Since data validation sets up the list, how can I check for duplicate entries?

Thanks in advance for any help.