Duplicates in my i-tunes library

I seem to have got myself in a right mess - I store my music on an external hard drive that I don't always have attached to the Laptop. Occasionally I've downloaded tunes from I-tunes store when the hard drive is not attached....so then I had music on the external hard drive AND on the C Drive.

Then I-tunes stopped recognising the hard drive, so I remapped it, and now I have duplicates of every song on there - half are mapped to songs on the external hard drive, and half are not mapped anywhere.

To make things more tricky it is the songs that are not mapped to anything that I need to keep because these are the songs that are in my playlists...I've started going through and deleting duplicates then remapping all the unmapped tunes but it is going to take the rest of my life to do this - is there any way to sort this mess??