While executing an upsert (https://www.zohoapis.eu/crm/v6/Leads/upsert), setting duplicate_check_fields to ["Website"] is not always working and, for some records, inserts another identical record. 
Creating another field of type "single line" and checking for duplicates on that one WORKS. Both fiels were marked as unique at the time of testing.
Example: 

            data: [{

                Last_Name: '-',

                fanculo: "test.com",

                Email: "test@test.test",

                

                Language: "lang",

            }],

            duplicate_check_fields: ['fanculo'],

        };