Change Or Remove An Sql Parameter

Hello.

I have a gridview that is connected to a sql datat srource witch gets a few columns to populate the GV with. I also have a drop down list where i enable the user to fill the gridview with the values that only match a specific category. What iv done is that i have a selected index changed event on my drop down list that does the following.string kategori = DropDownListKategori.SelectedValue.ToString();
Label1.Text = kategori;
SqlDataSource1.SelectCommand = "SELECT [ARTNR], [ARTTYP], [AKTIVITET], [DATUM], [KUND], [PLATS], [KOMMENTAR] FROM [ARTIKEL] WHERE [ARTTYP] = @ARTTYP";
SqlDataSource1.SelectParameters.Add(@"ARTTYP", kategori);
SqlDataSource1.DataBind();

Now when i launch the webpage and select a category (kategori) the gridview chanes and displays only the values that match that category but when i slect a new category Visual Studio throes me the following.

The variable name '@ARTTYP' has already been declared. Variable names must be unique within a query batch or stored procedure.

I thought that the parameter would get a new value but aparently when its set i cant declare a new value inside. What id like to know is how i can modify the above code so i can pass a new value to my select as soon as the drop down changes.