Currently, when the online giving form does not recognize a donor as an existing constituent record, it assumes that it is a new donor and creates a new record before the batch is even approved. This FREQUENTLY results in duplicates being created by RE that are not even recognized by the Duplicate Management feature. Donors will often put in their nickname, or put both spouses' names into the "first name" field on the giving form, and RE creates a new record because of these slight irregularities that the person approving the batch would have caught. Instead of creating a new record and highlighting them as a new donor, it should say something like "could not match to a record" and SUGGEST creating a new record. The person approving the batch can then match the gift to the existing record or create a new one after confirming the donor is actually new.
This already exists - you will need to go the Gift Overview page, on the right side, to Settings, and set the Online Data Review to review your matches. You can find more information here: https://webfiles-sc1.blackbaud.com/files/support/helpfiles/rex/content/on-data-review-tool.html?Highlight=online%20data%20review