Can Postfix prevent duplicate in RCPT TO address

There is a way how to do it, but not using Postfix itself.

If you are using a reasonably recent Dovecot with Pigeonhole implementation of Sieve, there is a special extension that can be used for on-delivery deduplication. In Dovecot older than 2.2.18, the name of this extension is "vnd.dovecot.duplicate" and it's disabled by default, in newer releases the extension is called "duplicate" and should be already enabled, but enabling it explicitly won't harm.

/etc/dovecot/conf.d/90-sieve.conf:

plugin {
  sieve = ~/.dovecot.sieve
  sieve_dir = ~/sieve

  sieve_before = /mnt/mail/users/global_sieve/deduplicate.sieve
  # sieve_before2 = /mnt/mail/users/global_sieve/antispam.sieve

  sieve_extensions = +vnd.dovecot.duplicate  # for dovecot < 2.2.18
  #sieve_extensions = +duplicate             # for dovecot >= 2.2.18
}

Then create the sieve script to handle the deduplication itself (you can adjust the filename as you see fit).

/mnt/mail/users/global_sieve/deduplicate.sieve:

require "vnd.dovecot.duplicate"; # for dovecot < 2.2.18
# require "duplicate";           # for dovecot >= 2.2.18

if duplicate {
    discard;
    stop;
}

Compile the script using sievec and make sure the script is readable by dovecot user. More in Dovecot docs.

If you are using Cyrus, there is a a duplicate message delivery suppression feature that can be enabled using suppress_duplicates = yes.

Local aliases go into

alias_database = hash:/etc/aliases (not in /etc/postfix/virtusertable) in the following format:

alias: destination. After that run newaliases.