POSTGRES UPDATE ON CONFLICT. Error cannot affect a row a second time (dupes) ONLY OCCURS IN FUNCTION. Not as query - postgresql

Question

I have a query in a function that does not seem to be returning any duplicates from my checks and if ran as a separate query... it works! If ran within a stored function, it gives the error ON CONFLICT DO UPDATE command cannot affect row a second time.
This makes no sense.
CREATE OR REPLACE FUNCTION rollups.compute_daily_rollups_every_hour(
start_time timestamp without time zone,
end_time timestamp without time zone)
RETURNS void
LANGUAGE 'plpgsql'
COST 100
VOLATILE
AS $BODY$
BEGIN
RAISE NOTICE 'Computing daily rollups from % to % (excluded)', start_time, end_time;
RAISE NOTICE 'Aggregating data into daily_rollup';
EXECUTE $$
INSERT INTO rollups.daily_rollup
SELECT
COALESCE(visitors, 0) AS visitors, COALESCE(total_dwell_time, 0) AS total_dwell_time, d.datestamp::date, doorway_id, customer_id, centre_id, postcode, gender, age_group, house_income, no_children, no_cars, shopping_frequency, marital_status, employment_status
FROM (
Select date_trunc('day', (current_date - offs)) AS datestamp
FROM generate_series(0, 365, 1) AS offs
) AS D
LEFT OUTER JOIN (
SELECT cv.datestamp,
round((date_part('epoch'::text, sum(cv.dwell_time)) / 60::double precision)::numeric, 2) AS total_dwell_time,
count(cv.sensor_id) AS visitors,
cv.doorway_id,
cv.customer_id,
cv.centre_id,
cv.gender,
cv.postcode,
cv.age_group,
cv.no_children,
cv.no_cars,
cv.marital_status,
cv.employment_status,
cv.shopping_frequency,
cv.house_income
FROM rollups.some_rollup cv
WHERE cv.dwell_time > '00:00:30'::interval
GROUP BY cv.datestamp, cv.doorway_id, cv.customer_id, cv.centre_id, cv.gender, cv.postcode, cv.age_group, cv.no_children, cv.no_cars, cv.marital_status, cv.employment_status, cv.shopping_frequency, cv.house_income
) AS t1
ON d.datestamp::date = t1.datestamp::date
WHERE d.datestamp >= $1 AND d.datestamp < $2
ORDER BY d.datestamp
ON CONFLICT (datestamp, doorway_id, customer_id, centre_id, gender, postcode, age_group, no_children, no_cars, marital_status, employment_status, shopping_frequency, house_income)
DO UPDATE SET visitors=excluded.visitors, total_dwell_time = excluded.total_dwell_time;$$
USING start_time, end_time;
END;
$BODY$;