In your database, does this mean that there is only one record in table 2 for every record in table 1?
If not, please give an example of what you want to prevent.
There are a number of approaches that can be used.
its an e-mail campaign db, it has a huge list of current subscribers and a list of unsubscribers. What I am wanting to do is, compare the unsubribers against the list of current subscribers to see if any have been entered into both so i can remove one.
They are set out so that one layout has the unsubs and the other has all the current ones so that it is easier to manage.
IF you define a relationship linking the two tables by email address, Only email addresses that occur in both tables will show a matching record in the related table. From the Suscribed layout Unsubscribed::emailaddress will not be empty if there is a matching entry in Unsubscribed. From the Unsubscribed layout, the same would be true for SubScribed::EmailAddress.
IF you put subscribed and unsubscribed contacts in the same table with a field to identify each, you can use a unique values validation field option to keep this from happening in the first place.