Something

postmap: warning: /etc/postfix/virtual/addresses.db: duplicate entry: "knowledgeshare.nl"
postmap: warning: /etc/postfix/virtual/addresses.db: duplicate entry: "demuldertjes.com"

/etc/postfix/virtual/addresses:
knowledgeshare.nl              DOMAIN
diamond@knowledgeshare.nl      diamond

knowledgeshare.nl              DOMAIN
@knowledgeshare.nl             diamond

demuldertjes.com               DOMAIN
corne@demuldertjes.com         corne

demuldertjes.com               DOMAIN
@demuldertjes.com              diamond