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From a pure ethics standpoint, you've had the use of the previous copies and disposed of them (albeit not willfully). Ethically you should buy it again.
If I buy a book and dispose of it through loss then someone else has the use of that book, hence the licensed material is in use by another person. It's not ethical to download a copy of the book as that would make two people having the use of the book for the price of one so to speak.
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