Rule causes baffling error

I'm trying to figure out why a rule gives me a uniqueness violation when I
try to do an update.

I have a table, "my_data", defined as:

create table my_data (
id INT8 not null default nextval('person_seq'),
effective_date_and_time TIMESTAMP WITH TIME ZONE not null default
CURRENT_TIMESTAMP,
expiration_date_and_time TIMESTAMP WITH TIME ZONE null default
'infinity',
user_name VARCHAR(255)null,
constraint PK_MY_DATA primary key
(effective_date_and_time, id)
);

I have a view, my_data_now, defined as:

SELECT
my_data.id,
my_data.user_name,
my_data.effective_date_and_time,
my_data.expiration_date_and_time
FROM my_data
WHERE my_data.effective_date_and_time <= 'now'::text::timestamp(6)
with time zone
AND my_data.expiration_date_and_time >=
'now'::text::timestamp(6) with time zone;

And I have this rule (among others):

CREATE OR REPLACE RULE upd_my_data_now AS
ON UPDATE TO my_data_now
DO INSTEAD
(
/* Update current record, and make it effective now. */
UPDATE my_data
SET id = NEW.id,
user_name = NEW.user_name,
effective_date_and_time =
('now'::text)::timestamp(6) with time zone
WHERE effective_date_and_time =
OLD.effective_date_and_time
AND id = OLD.id;
/* Insert a record containing the old values,
and expire it as of now. */
INSERT INTO my_data (
effective_date_and_time,
expiration_date_and_time,
id,
user_name)
VALUES (
OLD.effective_date_and_time,
('now'::text)::timestamp(6) with time zone,
OLD.id,
OLD.user_name)
)
;

This rule is supposed to (1) cause an update directed to the view
"my_data_now" to be made to the underlying table "my_data", (2) reset the
"effective_date_and_time" of that row to 'now', (3) insert a record
containing the old values into "my_data", and (4) expire that "old" record
by setting its "expiration_date_and_time" to 'now'.

But when I try to do an update against the view "my_data_now" with a query
such as:

update my_data_now set user_name = 'Suzy' where id = 1;

I get:

ERROR: duplicate key violates unique constraint "pk_my_data"

Presumably this happens when the rule tries to insert the new row. The new
row does indeed contain the "old" id and effective_date_and_time. However,
the rule is structured so that the current row's "effective_date_and_time"
gets updated to 'now' *before* the new row is inserted, making its value
different from the old "effective_date_and_time". So the uniqueness
conflict shouldn't occur.

I figure either there's some bug in my code that I can't see, or else the
PostgreSQL rule processor works in some way that I don't understand.

In either case, help!

~ TIA
~ Ken