Stored Procedure Select Query is returning same person Multiple times and just want 1 row???

hi,

I have gridview on my aspx page that displays data from a Stored Procedure with Dynamic paramters.
for example the Select Query returns a person based on
COuntryId
EthnicityId
AssignmentId
( example a person can undertake multiple assignments and so I have the following )
Assignment Table
Assignment Id
Name
ModelAssignment Table
AssignmentId
ModelId
This works fine but I  pass  a null as AssignmentId in the select query- then it shows the same person multiple times
one time for each entry that peron has in Assignments Table.
My stored procedure is below
thank you for any help?

SET ANSI_NULLS ON GO SET QUOTED_IDENTIFIER ON GO ALTER PROCEDURE [dbo].[model1]     @CountryId int,      @AssignmentId int,   &nb ...