The Installer for GarageBand 3 can optionally install the Apple Loops for GarageBand to any available volume or location on your system. If you upgrade from a previous version of GarageBand and elect to do this, your previously installed Apple Loops will not be moved or deleted, leading to possible duplication of some or all of the loops.
If this happens, you may delete the old copies of the loops after installing GarageBand 3. For an overview of the default locations for GarageBand loops, look here.
If you use any other applications that use Apple Loops, such as Logic Pro, Logic Express, Soundtrack Pro, or Soundtrack, you may wish to index the newly installed loops from GarageBand 3.