I haven't been writing anything here for three years now. It's a long time. Having reviewed some of the posts from 2008 and 2009 I find that's useful rereading what I have myself written before. Some of the knowledge didn't make into a long-term memory :-)

 Anyway, for today I discovered a new option for yum repository configuration files.
 includepkgs  This is a list of packages you want to use from a repository. If this option lists only one package then that is all yum will ever see from the repository.
This is useful if you want to add external repository because of only one package.

 exclude
excludes packages listed.
e.g. exclude=eclipse* kernel
This option is useful if you have a conflict among different packages in your repositories.

Note that if you have multiple exclude configuration lines, only the last one will be valid.
Same is true for any other configuration lines in yum.conf.
This is a limitation of yum, though not a huge one. Postfix, nagios can have multiple lines and they are all valid. Say, in nagios you can have as many configuration directories as you want. And all are valid:

cfg_dir=/etc/nagios/cfgs

cfg_dir=/usr/local/nagios/etc-common