How do I check whether the same string value appears within a range of cells

e.g.

A
1 JD
2 DF
3 AM
4 FC
5 RS
6 DF
7 JD

How do I get excel to check down column A1:A7 to see if any of the different
strings are repeated? In this example I want to return the answer "yes"
because JD appears in both A1 and A7. However, if this wasn't the case then I
would want excel to return the answer "no"

I am using excel 2007

Many thanks

JD