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Yes, because nobody's ever thought to have a Group named "Mind Over Matter". You are the sole proprietor of using that name in Destiny. That person must know you personally, saw what you were doing, and made their Group right before you did. Again, nobody has ever thought to utilize the known phrase "Mind Over Matter" as a name for their Group.
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Not sure what you mean by stolen. There is only one clan with your clans name, and you are the founder.