No Duplications?

Can anyone advise me on the following please.
I have built a very straightforward contact name and address DB to record all enquiries coming in to my business.
I don't want to enter any duplicates and I thought the best field to allow no duplications on was Address 1. Unfortunately this didn't work because I have three addresses all starting "The Old Coach House" but they are all in different locations. Then I thought of Post Codes, but we have enquiries coming in from countries that don't use Post Codes, like Ireland, so these are blank creating duplicates.
Has anyone got an answer to the problem, it's probably dead easy when you know how but being new to Access I can't see it.....HELP:(

Keith