How can I insert rows into a table from a select query that brings duplicate entries?

I am using PostgreSQL.

I have a table with columns:

tutor_id
subject_id

I want to populate this table with the result of the following select:

select tutors.id, subject_id from courses_subjects 
  join courses on courses.id = courses_subjects.course_id 
  join tutors on tutors.id = courses.tutor_id;

The point is that the above select brings duplicate entries. This breaks the insert

insert into tutor_subjects (tutor_id, subject_id) 
select tutors.id, subject_id 
from ...;

because tutor_subjects does not allow duplicate entries. The correct data that I have to insert appear to be:

select distinct(tutors.id, subject_id) from ....;

But I cannot use this select in combination with insert.

Any help?