Avoid duplicates

Avoid duplicates

Hi, 

I've got a subform with basic details like Client name, Suburb, Address etc.
It appears in two forms. 
I want there to be a single subform record for each Client name.
So if no subform record exists with the entered client name, create it.
But, if it does exist, update it without also creating a new subform record.

I think this is simple, but i'm having trouble figuring out where to implement this.

Thanks
Levi