Avoid duplicated rows when restoring data from pg_dumpall ??

Hi all,

I'm getting duplicated rows in some of my tables when I try to restore data
from a dump file generated with 'pg_dumpall'.
I use:
pg_dumpall -c -U postgres > mybackup.sql
to dump all the data, roles, etc. into a file.

Then I try to restore the data using:
psql -U postgres -f mybackup.sql

The only "way" I have found to solve this problem is deleting my database
before restoring the data, so everything is 're-created' in the restore
process without generating duplicated rows.

I thought, that the "-c" flag on pg_dumpall would force a DROP of any
previous data structures on the server but...it looks like it doesn't work
as I expected and ...when I restore data I get duplicated rows.

How can I get a "clean" restore automatically? I mean, getting exactly the
same data there was on the server at the moment I made the 'pg_dumpall'

Excuse me for my poor English, by the way.

Thanks in advance,