Delete all columns in a datagridview

I have a datagridview which gets loaded from a datatable. I also added manually a combobox columns. When I clear the datagridview and reload I dispose the datatable and make a fresh copy of it and that's how the columns in the datagridview clear and reload. The problem is that the combobox column doesn't clear so when I reload it it has this column 2 times. I can't do datagridview.columns.clear because it's loaded straight from a datatable. How can I fix this problem?

Here is some of the code:

            if (dsInvoicingFilter !=null) dsInvoicingFilter.Dispose();

            dsInvoicingFilter =newDataSet();

           if (bs != null) bs.Dispose();

            bs =newBindingSource();

           switch (invoiceType)

            {

               case"work orders":

                    dsInvoicingFilter =Data.GetDataSet("WorkOrders_Select");

                   DataGridViewBinding.BindWithData(true, dsInvoicingFilter.Tables[0], dgvInvoices, lblEntryCount, bs);

                   DataTable dt = Data.GetDataTable("Employees_Select");

                   DataGridViewComboBoxColumn cmbColumn = newDataGridViewComboBoxColumn();

                    cmbColumn.Name ="PickedBy";

                    cmbColumn.HeaderText ="Picked By";

                    cmbColumn.FlatStyle =FlatStyle.Flat;

                    cmbColumn.DataSource = dt;

                    cmbColumn.DataPropertyName ="EmployeeID";

                    cmbColumn.ValueMember ="EmployeeID";

                    cmbColumn.DisplayMember ="Name";

                    dgvInvoices.Columns.Add(cmbColumn);

                    dgvInvoices.Columns[0].SortMode =DataGridViewColumnSortMode.Automatic;

                   break;

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