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QuickBase relies on email addresses to identify user accounts. When QuickBase user accounts are created, they are initiated and confirmed via an email address. Sometimes a user may inadvertently create two or more accounts, each having a different email address. This can easily occur when users change companies and email addresses. QuickBase recognizes these as completely separate accounts, therefore they have their own set of applications which they can access. If a user tries to change the email address to an address that is already in use by another account, QuickBase will not permit the change. Because QuickBase accounts can not be merged, the original account with the desired email address must be changed to something different, such as firstname.lastname@example.org. This can be done by logging into this old account and changing the email address as described here: https://www.quickbase.com/help/default.html#change_my_user_profile.html
Once this change is made, the second account email address change will be permitted.