MariaDB 10.3

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create table t (id int, user_id int, project_id int);
insert into t values
(17373,      6336,       1001),
(17374,      6336,       1001),
(17375,      6336,       1001),
(529044,     6336,       1001),
(17305,      6342,       1003),
(17350,      6318,       1003),
(17388,      6318,       1003),
(811062,     6342,       1003),
(811063,     6342,       1003),
(1552793,    6342,       1003);

Records: 10  Duplicates: 0  Warnings: 0


select
    project_id,
    count(distinct user_id) usr_count
from
    t
group by
    project_id
having
    count(distinct user_id) > 1;


project_id
usr_count


1003
2


SELECT MIN(id) id, user_id, project_id
FROM t
JOIN ( SELECT project_id
       FROM t
       GROUP BY project_id
       HAVING COUNT(DISTINCT user_id) > 1 ) tt USING (project_id)
GROUP BY user_id, project_id;


id
user_id
project_id


17350
6318
1003

17305
6342
1003