Data Entry: Managing Duplicates

Data Entry: Managing Duplicates

Hello,
 
I'm hoping someone can help me with this. I am building a database with several people inputting data. I would like to include a field (I'm thinking their unique email address) where if there is a duplicate, it looks up the existing record so that the second user can edit it (to say that its their client as well). I don't want two records... but I'm stuck trying to figure out the best solution.