Error loading data from one table into another - SQL Server 2014

I have the following stored procedure which I use to fill the Resources table. To load the Resources table, I get the IdEntity that I bring from a datagrid that contains a list of accounts. The problem is that when loading the data in the Resources table, if I select 350 accounts, I upload 350 records regardless of whether the record already exists or not.
I leave the tables and the stored procedure:

Resources
-----------------------------
ResourceId(int)            
ResourceName(varchar(max))  

Carteras  
-----------------------------
IdCartera (int)  
NombreCartera (varchar(max))  
estado (varchar(max))  
IdEntidad (varchar(max))  

Entidad  
-----------------------------
IdEntidad (int)  
NombreEntidad (varchar(max))  
estado (varchar(max))  
Alias (varchar(max))  


CREATE PROCEDURE [dbo].[sp_CargarTablaResources]
@pIdEntidad int
AS
BEGIN

SET NOCOUNT ON;

    if exists(
        select ResourceId, ResourceName from Resources
            where ResourceId = @pIdEntidad)
    Begin
        raiserror('Err -998: El registro ya existe.', 1, 10)
        return -998
    end
    Else 
        begin
            insert into Resources (ResourceID, ResourceName)
            select Carteras.cart_Id, Carteras.cart_Codigo from Carteras
                where Carteras.enti_Id = @pIdEntidad
        end
END

The result I get is:

I have this method in the Data layer which is executed for each account that I have loaded in the datagrid to verify each IdEntity, then I am loading the data without evaluating if the record already exists or not:

public static void LlenarTablaResources(int IdEntidad)
{
  SqlConnection con = new SqlConnection();
        con.ConnectionString = "Data Source=MiServidor;Initial Catalog=MiProyecto;Integrated Security=True";
        SqlCommand cmd = new SqlCommand();
        SqlDataReader dr;
        cmd.CommandText = "sp_CargarTablaResources"; 
        cmd.CommandType = CommandType.StoredProcedure;
        cmd.Parameters.Add("@pIdEntidad", SqlDbType.Int).Value = IdEntidad; //("@IdEntidad", SqlDbType.Int).Value = IdEntidad;

        try
        {
            cmd.Connection = con;
            con.Open();
            dr = cmd.ExecuteReader();
        }
        catch (Exception ex)
        {
            throw ex;
        }
}