Even after clearing the certs from puppetmaster and client, if you are getting below error on your puppet client
Error 400 on SERVER: A duplicate resource was found while collecting exported resources, with the type and title Host
then its because you have messed up with certs badly.
Here is what you need to do.
Check the certs name from ls -al ${PUPPET_HOME}/ssl/ -R
once of the cert above has multiple hostname in it. Find all the hostname that comes as part of above command and delete all of them from puppet master.
once done clean the ssl folder from client
rm -rf ${PUPPET_HOME}/ssl/
and run the puppet agent.