the duplicated music files are in your itunes folder, go to music --> itunes --> itunes music --> and delete all those
Huh? The OP says that their Photos are on an external, NOT his music. Doing this would wipe out the only copy of his/her music.
To the OP:
If you only have 40GB of music, consider moving that to an external (at least temporarily) and consider, as others have suggested, upgrading the internal HD. SATA 2.5" drives are dirt cheap (compared to older PATA 2.5" drives).
If you have a current backup, consider checking the directory structure using Disk Utility. I've seen circumstances where directory corruption suddenly allows a small file to think it's consuming several GigaBytes of space. You can do a "Verify" without booting from your original install Disc.
Other things might be overgrown log files due to errors being logged frequently (or TOO frequently).
Until you free up some space, be very careful how many Apps you run. Reboot and check the free space. If you find you have a lot more, this is the Virtual Memory swap files. They will grow in size the more programs you run simultaneously as you run out of "real" RAM.
Too long a post, sorry 'bout that.

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