root@puppetmaster:/tmp# cat nameclash.pp
class puppetmaster {
        err("puppetmaster included")
}

class puppetmasterx {
        err("puppetmasterx included")
}

node puppetmaster {
        include puppetmaster
        include puppetmasterx
}

root@puppetmaster:/tmp# puppet --verbose nameclash.pp
err: Scope(Class[puppetmasterx]): puppetmasterx included
root@puppetmaster:/tmp#