New to back up's

I am simply new to backups; have not performed a new back since I first bought my iMac this summer. Now I have over 800GB of music, garageband files, and movie file's on my iMac. Most all of my music for example in my iTunes is already backed up on my time capsule. Does this mean that it will require another 800GB's of space on my time capsule when I back? That I will essentially end up with two copies of all of the itunes music for example?

Thanks for any help.