VB Access duplicate record Message

Hi

I am trying to write code that recognises duplicates in a record and pops up with a MsgBox to let you know that it has already been added..

I have written some myself but can't get anywhere!

The table which hold the data is called tblRecruitmentData and the field is PostCode. The field in the form is called txtPostCode.

Can you help me at all?

Thanks

Clare