Autovacuum firing up during my manual vacuum on same table

Greets,

I just noticed something odd:  I'm busy with a manual vacuum on a table and an
autovacuum keeps firing up as well.  Thinking this looks rather weird, I
pg_cancel_backend() the autovacuum process:

current_query | vacuum analyze page_citation_text;
age           | 11:34:10.759279
...
current_query | autovacuum: VACUUM ANALYZE public.page_citation_text
age           | 11:33:15.824014


However, Pg keeps firing up the autovacuum:

current_query | vacuum analyze page_citation_text;
age           | 11:46:57.245568
...
current_query | autovacuum: VACUUM ANALYZE public.page_citation_text
age           | 00:11:50.571


It seems counter-intuitive to have two vacuum procs running on the same
table...  how is this possible?

I'm trying to vacuum the table as quickly as possible so a manual vacuum seems
to be in order as my understanding (and experience) is that the autovac is a
hell of a lot slower to mitigate impact on general performance.

Anyway, is that autovac duplicating work or locked out and waiting?

Thanks
Henry