Errors recovering database

Hello,

I'm having issues restoring a dumped database with the following commands:

Dumping:
pg_dump -p -U'postgres' -C -f ./dumpbkp mydatabase
tar -cvzf dumpbkp.tar.gz dumpbkp

Restoring:
cat dumpbkp.tar.gz | gunzip | psql -U'postgres'

The database is quite big, so it takes a lot of time to restore. The process
outputs some normal messages and, in the end, after a lot of time, it prints
out some strangers errors like "duplicate key", "constraint already exists",
and other similar...

at the end, I've got a database 10 GB smaller than the original, which
clearly indicates that some data was lost.

The versions are 8.4.5 (original) and 8.4.7 (restored).

What should I do? Am I doing something wrong?

Thanks in advance.

Henrique Teófilo