Cannot insert a duplicate key into unique index - Mailing list pgsql-novice

I'm trying to debug some problem in my database that is resulting in
an error of the form "Cannot insert a duplicate key into unique
index".  The insert statement that is producing this error does not
include a value for the pkey field in question (this field is of type
SERIAL).  I imagine that somehow the counter associated with this
field got messed up, so that it is mistakenly generating a value that
has been used already.  How can I straighten it out?

Thanks!

kj