It depends what you mean by "upgrade" but there is no such thing as a major upgrade where the ProductCode stays the same. Keeping it the same is not a constraint, it's against what a major upgrade does. You should say why you believe it's a constraint to keep it the same, and if you decided it's a solution to a problem you haven't told us about.
The error message is from a WiX util CreateUser custom action of some kind. You haven't specified what you have changed in your MSI file, but if you are just running your new MSI and it's got the same ProductCode then you are probably doing a repair, and that repair is running the CreateUser again and telling you it already exists. You should supply more info about your CreateUser.
You'll also need to say what kind of upgrade you are doing (major, minor?) and if it's minor then what is your command line?
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