At the basics, you would write a producer that consumes from one topic and produces to another.
MirrorMaker is what you are looking for.
Yes, MirrorMaker is not putting a limitation on remote vs local cluster. It is designed for remote clusters because there is almost no need to do it locally. If you are mirroring a topic locally, you must rename it, and if you are going to rename it, then you have consumers/producers using data in both topics?
You are replicating data within the same cluster for little gain while your consumers/producers can easily be configured to use the correct topic(s).