これらの手順に従って重複ユーザーの名称変更には成功しましたが、再インデックス作成時にこの問題に遭遇しました。
@sam はそのスレッドで「はい、重複行を削除（nuke）してください」と提案されましたが、再インデックス作成時に現在までに膨れ上がっている警告やエラーメッセージの文脈で、それが何を意味するのか分かりません。
REINDEX SCHEMA CONCURRENTLY public;
WARNING: cannot reindex invalid index "public.index_incoming_referers_on_path_and_incoming_domain_id_ccnew" concurrently, skipping
WARNING: cannot reindex invalid index "public.incoming_referers_pkey_ccnew1" concurrently, skipping
WARNING: cannot reindex invalid index "public.index_incoming_referers_on_path_and_incoming_domain_id_ccnew1" concurrently, skipping
WARNING: cannot reindex invalid index "public.index_incoming_referers_on_path_and_incoming_domain_id_cc_ccnew" concurrently, skipping
WARNING: cannot reindex invalid index "public.index_incoming_referers_on_path_and_incoming_domain_id_c_ccnew1" concurrently, skipping
WARNING: cannot reindex invalid index "public.incoming_referers_pkey_ccnew2" concurrently, skipping
WARNING: cannot reindex invalid index "public.index_incoming_referers_on_path_and_incoming_domain_id_ccnew2" concurrently, skipping
WARNING: cannot reindex invalid index "public.incoming_referers_pkey_ccnew_ccnew" concurrently, skipping
WARNING: cannot reindex invalid index "pg_toast.pg_toast_19250_index_ccnew1" concurrently, skipping
WARNING: cannot reindex invalid index "pg_toast.pg_toast_19250_index_ccnew2" concurrently, skipping
WARNING: cannot reindex invalid index "pg_toast.pg_toast_19250_index_ccnew_ccnew" concurrently, skipping
ERROR: could not create unique index "index_incoming_referers_on_path_and_incoming_domain_id_ccnew3"
DETAIL: Key (path, incoming_domain_id)=(/votes/, 1165) is duplicated.