To be fair, this isn't exactly uncommon in the history of business.

If I remember correctly, in Drucker's Innovation and Entrepreneurship, this exact phenomenon is discussed as a counterpoint to the first-mover advantage. It is often easier to let someone else do the hard work of verifying that the market for a particular class of product exists. Once you've seen what went well for them and what didn't, you can learn from their successes and mistakes and use that data to one-up them.

From the perspective of Turntable.fm, it isn't necessarily bad news either. If your product is successful enough to inspire someone to compete with you, it might just be a positive indicator of the viability of the market you are trying to serve.