About a week ago I changed my clan name and clan ID. After a quick relog the name updated but the ID did not. I logged out and closed Destiny, then came back to Bungie.net > My Clan > Admin Tools and I deleted the ID, rewrote it and pressed save, but I got a message saying "The clan name is already used. (691)". Logged back into Destiny but the Clan ID still did not update.
I have repeated this process several times during the week, and I get the same result every time.
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If the error you're receiving says it's already in use, then it means someone else is already using it; you'll need to choose something different.