how to add more than one cd into my music library

In Banshee (which is the default music playing software starting with Ubuntu 11.04 -- you didn't say what software you're attempting to use to import the music, nor what version of Ubuntu you're running, so I'm just guessing here), you can put a music CD in the drive, click on it on the left side of the screen (it appears near the bottom), then click Import CD near the upper-right corner of the screen. You should be able to do this for any number of CD's (as long as you have available hard disk space), and it should be able to import all of them, and it should not attempt to overwrite the files from one with files from the other.

If you are using some other procedure to add CD's to your music library, then you might want to use the above procedure (though if you're using other music playing software, you should probably use that to import the CD instead). If the above information doesn't tell you what you need to know, please provide more details about what you are doing and what is happening; please also provide the complete and exact text of the warning message that you are presented with about overwriting already-ripped music.