Ok. I think I understand the issue now.
You have two options
1) Your should set the coalesce on user id instead of the object GUID.
Because if AD team created a new account with a new GUID, but username is same, based on the coalesce, transform map will try to create a new user account in servicenow. But it will fail to do so, because an account with same user name already exists. User name is the unique key in User table.
2) You should rename the user account after an account is terminated. for ex, we rename our user accounts to username$todays date. So that, when a new account is created with the same username, there is no clash and transform map can create the account successfully.
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