SQL Duplicates

I need some help.

I have created a database that looks like the following:

FirstName Table link to Main Table.

I have created a Stored procedure that looks like this:

Create procedure dbo.StoredProcedure

(

   @FirstName   varchar(20)

)

Declare FirstNameID int

Insert Into Main Table

(

FirstName

)

Values

(

@FirstName

)

Select @FirstNameID = Scope_Identity()

How could I redesign this to check if a value exists and if it exists then simply use that value instead of creating a new duplicate value?

 

Thanks!!