ORA:30926: unable to get stable set of rows in the source table

hi i have the frollowing merg sql how can i avoid the duplicate value from source table to update the target table,or is the another way i can work around this

merge into tmp_cal_obj e1
          using cal_book e2
          on (e1.obj_name = e2.obj_name)
          when matched then update set e1.description = e2.description;

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how can i do this to above query
SELECT *
FROM (
SELECT col1, col2, col3,
count(*) over(partition by col1) cnt
FROM source_table

or this s

merge into cms_equipment eq
     using (select *
              from (select row_number () over (partition by trim (stg.equipment_id), vpf.vpf_company_nr order by null) rn,
                           vpf.vpf_company_nr,
                           trim (stg.equipment_id) as equipment_id,
                           stg.equipment_type,
                           nvl (stg.installation_date, stg.effective_date) as installation_date,
                           stg.projector_model,
                           stg.ti_software,
                           stg.projector_serial,
                           stg.server_software_version,
                           stg.certificate_number,
                           stg.mb_firmware_version,
                           stg.mb_link_encryption,
                           stg.watermarking,
                           stg.system3d,
                           stg.server_model,
                           stg.server_serial
                      from cms_equip_staging stg, cms_vpf_company vpf
                     where upper (trim (stg.vpf_company_name)) = upper (trim (vpf.vpf_company_name)) and upper (trim (stg.data_load_status)) = 'NEW')
             where rn = 1) estg
        on (upper (trim (estg.equipment_id)) = upper (trim (eq.equipment_id)) and eq.vpf_company_nr = estg.vpf_company_nr)
when matched
then
)
WHERE cnt = 1

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so i can ignore duplicate values