Folks who are planning to cultivate an elibrary should be aware that their ebooks may need to be replaced as technologies evolve.

That may come as a disappointment to Kindle fans: After shelling out more than $300 for a sleek little ebook reader, Amazon (Stock Quote: AMZN) Kindle users could have to pay twice for books they’ve already bought due to unclear licensing agreements.

Users who shop at the Amazon Kindle store can only download each book a certain number of times – a number that depends on the book and isn’t disclosed, according to The Consumerist. If you exceed the allowed number of downloads of a certain book, for example to a few different devices, you’ll have to pay for the book again.

Yet the Kindle store doesn’t disclose this digital rights management limit to shoppers on its Web site. In fact, it says that as long as your ebook readers are on the same account, they can share the same book, The Consumerist notes.

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And while Amazon calls the iPhone “a perfect companion for your Kindle,” if you’ve exceeded your allowed number of downloads, it could make reading an ebook on your iPhone just another added expense.

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