Curation menu > Organization with new duplicates.
In this section, you will find a list of orgs and by clicking on each of them will get a list of possible duplicates (orgs with a certain degree of similarity detected by the automated process). The curation work consists in accepting or rejecting this similarity.
If you want to enter duplicates that were not found by the algorithm, you must open the single org page and enter the duplicate in the designated section.