Before creating a new record, use various searches to be sure there is no record in the database for the entity, either assigned or provisional. See the Searching page. There is also extensive information about searching through the "Help" link at the top of the ISNI page.
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In some cases, a provisional record can be made upgraded to "assigned" status by contributing more information to the record. In other cases, it is necessary to use the WinIBW client to change the status of the record.