If he was double tagged, you can read his visual tag and order a "replica tag" of eid type with the identical herd and individual number to the other tag, and record the replacement of that tag in your holding register.
Otherwise I believe you would have to replace both tags with your tags, but I believe they would have to be red replacement tags.
Each country (England, Wales, Scotland) has slightly different rules, so double check with your local Animal Health office to be sure.