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