postfix and mysql support in FC6

[Alexandru Ciobanu]

> yumdownloader --source postfix
> Loading "installonlyn" plugin
> Loading "installonlyn" plugin
> Repository extras-development-source is listed more than once in the
> configuration
> Repository extras-development-source is listed more than once in the
> configuration
.....

It's pretty self explanatory. You have those repos listed more than once.
See your yum.conf and your *.repo files.