I get these errors when building a mailing list: 

Error at row 0: "duplicate key value violates unique constraint "mailing_subscription_unique_contact_list" DETAIL: Key (contact_id, list_id)=(2330, 9) already exists. "

Error at row 1: "duplicate key value violates unique constraint "mailing_subscription_unique_contact_list" DETAIL: Key (contact_id, list_id)=(2331, 9) already exists. "

Error at row 2: "duplicate key value violates unique constraint "mailing_subscription_unique_contact_list" DETAIL: Key (contact_id, list_id)=(2332, 9) already exists. "

What should I change?