Need help understanding error message

In a database table I have these rows:
# select * from contacts where person_nbr=6;
person_nbr | contact_date | contact_type | not
es | next_contact
------------+--------------+--------------+---------------------------------------------------------
---------------------------------------------------------+--------------
6 | 2018-04-05 | Phone | Left message @ 9:39.
|
6 | 2019-05-14 | Phone | He can call me if issues do come up. |
6 | 2021-08-17 | Email | Sent message 2.
| 2021-10-06
6 | 2021-07-23 | Email | Sent message 1.
| 2021-07-28
6 | 2021-07-28 | Email | Sent message 1.
| 2021-08-16
(5 rows)

When I try to update this table with:
(6,'2021-10-26','Email','message 3','2012-11-16'),
psql reports this error:
psql:insert_into_contacts.sql:31: ERROR: duplicate key value violates unique constraint "activities_pkey"
DETAIL: Key (person_nbr, contact_date, contact_type)=(6, 2021-10-26, Email) already exists.

The PK for this table is:
PRIMARY KEY, btree (person_nbr, contact_date, contact_type)
so there should be no conflict as the contact_date in the update is not
already in the table.

I don't understand the error as it's not occurred before when updating this
table.

Insight needed,

Rich