duplicate key value violates unique constraint "index_rs_evaluations_on_reputation_name_and_source_and_target"

[thebravoman]

The following error occurs on postgre running on heroku.

PG::UniqueViolation: ERROR: duplicate key value violates unique constraint "index_rs_evaluations_on_reputation_name_and_source_and_target"
DETAIL: Key (reputation_name, source_id, source_type, target_id, target_type)=(watched_points, 135, Episode, 3774, User) already exists.
: INSERT INTO "rs_evaluations" ("created_at", "data", "reputation_name", "source_id", "source_type", "target_id", "target_type", "updated_at", "value") VALUES ($1, $2, $3, $4, $5, $6, $7, $8, $9) RETURNING "id"
app/models/episode.rb:70:in `block in update_watched_points'

This is the code of the method being called:

def update_watched_points percentage, user
    to_be_given = calculate_some_value
    ActiveRecord::Base.transaction do
    current_points = self.evaluation_by :watched_points, user
        current_points = current_points.to_f
        if current_points < to_be_given
            user.add_or_update_evaluation :watched_points, to_be_given, self
            self.add_or_update_evaluation :watched_points, to_be_given, user
        end
    end
end

Do you know what might be causing it?