Trouble incrementing a column

Greetings,

I am using PostgreSQL 10.10.  I am having trouble incrementing a column for
reasons I can't see.  It's probably some basic SQL thing.  Your help is
appreciated.

create table my_table (
    listid char(36) not null,
    seq smallint not null,
    item varchar(4096),
    primary key (listid, seq)
);

insert into my_table (listid, seq) values ('abc', 1);
insert into my_table (listid, seq) values ('abc', 2);

-- the following works some of the time
update my_table set seq=seq+1;

-- the following doe not work for reasons I do not know
update my_table set seq=seq+1 where listid='abc';

What I get is a duplicate primary key.  I wouldn't think I'd get that
because I'd think the whole thing is done in a transaction so that
duplicate checks wouldn't be done till the end (essentially).

Is there a clean way to do this?

Thanks!

Blake McBride