I just submitted the following question to the EE support staff, and they suggested I submit my problem here:
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My mailinglist does not reject or filter out duplicate email addresses anymore.  (Perhaps it has something to do with the fact that I am using Solspace’s Free Form module, with the ‘mailinglist_opt_in’ parameter set to ‘no’.)

Does anyone know of a way to fix this?
Or of a quick way to eliminate duplicate email addresses?
Or, in case nothing can be done about the duplicates, is there at least a way of filtering them out when I send mail?

Thanks, Jules

EDIT:

Jules,

After some further testing, I’ve been able to duplicate this issue.
Strange though, as it seems intermittant. This testing included duplicating it WITHOUT the mailinglist_opt_in parameter as well.

I managed to only get duplicates with the “default” mailing list. Custom/New ones did not have that issue.... are you using this on the default mailing list?

I will queue this up for the next release

Thank you for your patience!

Thanks for working on it Pie Man.  My duplicates occur in a custom mailing list.  (As a rule, I try not to touch anything default so I can go back and look at it later.)

I too am noticing duplicate entries...here is my code I am using: