How do I delete double ups of songs on my iTunes library?

On my iPod, I have double ups of songs I have bought from iTunes, and when I connect it to my iTunes library on my laptop, the double ups don't show but they are still on my iPod? I'm so confused! So I can't click File and Show Duplicates because the double ups don't show on my laptop, only my iPod. Please help?How do I delete double ups of songs on my iTunes library?
If you have them save to like a playlist like "90's music" or classical music, then click edit and select the songs you want to remove. This is all done on your ipod