Each efile return should have a UNIQUE document control number (DCN). The DCN shouldn't be similar to tax payer's S.I.N. When EFiler contacts CRA EFILE helpdesk, a DCN will be needed for inquiry about a specific filing.
You can change document control number at the bottom of form T183. If you use an used DCN a for new tax return efiling, an error code 184 plus error clue 00000078 will be returned.