When you go to log in with your Steam account and it asks you to create a new account, it's because the Steam ID linked to this one is different, and that's part of the proof of it. If it had been the same as the one you had linked here, it would have just logged you into your usual Bungie.net profile, and it wouldn't have asked you to create a new one or link existing. Because the Steam ID you have linked to the current one cannot be unlinked, the link existing option won't work for you, since this Bungie.net profile already has a Steam ID linked to it. Unfortunately, your only option is to log in with the Steam ID you, yourself, already have linked to this profile, and you won't be able to use the Steam ID you regularly use and log in with in order to set up Cross-Save, since it cannot be linked to this profile, nor can the XBL ID linked to this profile be removed and linked to a different profile. If you wish to play on Steam using your Xbox save data, you'll need to use Steam's 'trouble logging in' link on the login page to go through the steps of recovering the account. You'll get to the point of entering a code from your email, and it will then show you each Steam username associated with that email. Ideally, you'll have 2, this one, and the one you normally use. You'll need to log in on Steam with that username, and it's most likely using the same password you normally use. There's no other options available to you if you want to set up Cross-Save and play with your Xbox save data on Steam. You can choose to continue with your normal Steam ID with a new Bungie.net profile and using new save data if you wish, but you won't be able to move or merge your Xbox save data into that or have access to it. Most likely, what caused this, was when you initially linked the Steam ID to your Bungie.net profile, you logged in with your email instead of your Steam username, so it linked a separate base account that you may not even know you had.