So when I published, I began to learn that as an author "typical" reader behavior is not something I can actually participate in. I have to be careful of how my words and actions will be perceived. "Any" negative comment that I make can portray me in a bad light. What does that mean for me? Well, I'm no longer able to write negative book reviews for one. No matter how dissatisfied I am with a book, I can't say anything about those feelings or thoughts. All I can do is review the ones that I have a positive review to share. I also feel the need to take a step back from religious or political conversations. Believe me, at times it's not easy to keep my mouth closed. I'm a naturally opinionated personality and when I feel strongly about something I want to share my thoughts. It's sort of the American way, you know?
Freedom of speech is the political right to communicate one's opinions and ideas using one's body and property to anyone who is willing to receive them. The term freedom of expression is sometimes used synonymously, but includes any act of seeking, receiving and imparting information or ideas, regardless of the medium used. ~Wikipedia
Freedom of speech is protected in the First Amendment of the Bill of Rights and is guaranteed to all Americans. ~https://www.aclu.org/free-speech
The situation: You enter a contest and win. To enter the contest, in some way you have supported the entity giving away the item. The item you win is sent to you in such a way that you are able to return the item.
For example, you win an ebook from an author. After you check your Kindle you find that you already own the book in question. So, when the gifted book arrives in your inbox from Amazon you return the gift for a credit and use that credit to buy another book from the same author.
That makes me wonder about the definitions of "gift" and "prize" (see above and below).
A prize is an award to be given to a person, a group of people, or organization to recognize and reward actions or achievements. ~Wikipedia definition
That being said, the following is my opinion.
In the situation above, I feel that if you won the prize in a giveaway and you already own the book, exchange it for another book. Preferably from the same author, but if nothing is available, then from another. Personally, I would then make sure to share the book info of the book I returned on Facebook, Twitter, or other social media. Do a bit to help that author earn the income and "purchase numbers" that they lost by your return.
You should understand that when an author gifts you a copy of the book, they have to pay that money, but they then turn around an earn their royalty back -- a few months down the road they will earn that money back, it takes time for royalties to be processed and paid. So, they are not paying the full price, but they are paying something for you to earn that prize.
That being said, if the author created the giveaway and specifically stated that it was for new readers (i.e. those that had not read the book) only or has specific entry qualifications that you did not complete. In this case, I feel that you really should contact the author and offer to return the ebook. They might decide to let you keep the credit, but it should be their choice.
So, a gift/prize is a freely given item - therefore, once I send it to you, you have the right to do what you want with the gift. Obviously, I"m not speaking about pirating, but if you have a book and want to send it on to a friend (at least from me) that is A-okay. If I gift you through Amazon, trade it in for a book of your choice. I hope that if you get to know me through the giveaway that you will follow me and purchase my future books. That's the point of a giveaway. Oh, and if you already have my book, please leave a review. Your feedback will help my future books be the best they can be.