We'll leave this open for more votes, although it would be hard to implement something like this without a lot of false positives interrupting your opt-in flow.
While we see how much interest this generates, these tips might help for others that find this feature request via search.
OntraForms: OntraForms pre-fill the name and email fields if the contact filling out the form is cookied. The contact would have to overwrite the email address to enter a new one.
Duplicate records with the same email address: These only happen if they are entered manually, or via the API (using the wrong API call to add the contact). The proper API call to make is "Add or Update" the contact (instead of just blindly using "Add Contact" as some scripts do).