I assume this is done through validation I just dont know how?

I have column B rows 1 to 500. I enter customer order numbers in on a daily
basis, I want it to error out if I attempt to enter the same order more than
once.

For instance if I am on row 400 and I try typeing in order 123456 and it
exists on row 286 it warns me or alerts me that it was already used?

Any Ideas Would Be Great... Thanks...

Sean