If you're only looking to add new Contacts and never update existing ones, you can just leave the else block out entirely. This code is in here assuming that if a Contact subscribes to the list through whatever flow you have, you want to make sure that the contact is added to that list (in addition to any other lists they may be on already). That's the only thing this code does and can easily be left out.
A best practice recommendation is that if the Contact is interacting with you directly, you message them that they are already on the list though if you aren't planning on updating them and giving them a success message. If the Contact isn't interacting directly with your app, this isn't really important.
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