Thank you for bringing your question to the Community!
The cornerstone of your contacts is their email address. If you change information that is associated with an address the contact will be considered "Updated". You can have as many contacts as you like with the same exact name but you can only have a unique email address once in your account. In your case, you would simply have 2 Mary Smiths on your account, one for each address.
I sure do hope that this helps to explain but please let us know if you have additional questions. Thank you for using Constant Contact, have a great day!
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