为什么我的唯一索引在 Postgresql 中不起作用

我为创建了一个表来存储用户的休假数据.之后，为多列设置唯一索引，但当EXEC插入SQL时，它会再次复制数据

这是我的DDL:

create table day_off_movements
(
    id                 bigserial
        primary key,
    user_id            bigint
        constraint fk_day_off_movements_user
            references users,
    proxy_user_id      bigint
        constraint fk_day_off_movements_proxy_users
            references users,
    reason             text,
    day_off_start_date timestamp with time zone,
    day_off_end_date   timestamp with time zone,
    used_days          bigint,
    created_at         timestamp with time zone,
    updated_at         timestamp with time zone
);

索引:

create unique index idx_day_off_all
    on day_off_movements (user_id, proxy_user_id, day_off_start_date, day_off_end_date);

控制查询:

SELECT user_id,proxy_user_id,day_off_end_date,day_off_start_date,count(*)
                 FROM public.day_off_movements t
GROUP BY user_id, proxy_user_id, day_off_end_date, day_off_start_date

输出为json:

[
  {
    "user_id": 961,
    "proxy_user_id": null,
    "day_off_end_date": "2020-07-29 00:00:00.000000 +00:00",
    "day_off_start_date": "2020-07-27 00:00:00.000000 +00:00",
    "count": 3
  }
]

我哪里不对劲？ 谢谢你的帮助.