I have just installed MAMP. And I have just now realized that I already have a MySQL server and a database that runs under it. Do I now have two MySQL servers? Will they interfere with each other (and if yes, what would I need to do to have the existing database to work with the MAMP version)?
I wouldn't anticipate any problems. I've seen those configurations work just fine. The only concern I see is that the built in versions of the software get updated automatically by Apple, and the versions you installed will not. However, Apples updates include the latest fixes more slowly, so if you are diligent in installing them, yours will be up to date a little sooner. If you are not, the Apple versions are easier to keep updated as the updates are almost completely automatic.