In apps/redmine/app/models/project.rb, there is User.active.joins(:members).where("#{Member.table_name}.project_id = ?", id).distinct. which will be translated to SELECT DISTINCT users.* FROM users INNER JOIN members ON members.user_id = users.id WHERE users.status = 1 AND (members.project_id = ?);

Actually DISTINCT is not necessary due to the uniqueness constraint in member.rb: validate: user_id, uniqueness: {:scope => project_id}, which indicates that for member1(member_id1, user_id, project_id) , and member2(member_id2, user_id, project_id), there is member_id1 = member_id2. Or else the uniqueness constraint will be violated. As a result the join result will not return duplicate users.