Avoiding Duplicate entries - from a drop down

[Guest]

Hi there, I am trying to create a rule to prevent duplicate 'entries' from
being selected, but is unsuccessful in doing so. Basically, my worksheet
allows a user to select an entry from a validation list (drop down). The
problem with this set up is that there is a high probability that another
user would select the same option. How could I disallow or warn the user(s)
that a similar entry already exists?

For example, the drop down consists of colour selections (red, yellow,
green, blue, etc.). I only want RED to be selected once. If user 2 selected
RED again, I want a prompt warning him/her that RED already exists.

By the way, this only impacts one column.

How could I do create a 'condition' warning the user that a similar entry
already exists or even preventing them from selecting a pre-existing entry
again?

 

[Biff]

See if this helps:

http://contextures.com/xlDataVal03.html

Biff

 

[Guest]

Thanks for your help. This formula was actually more helpful in terms of what
I wanted to do:

=IF(COUNTIF(NAME_1,A1)>1,TRUE,FALSE)

Your assistance is greatly appreciated!