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If I send an email blast two two lists at the same time and someone is subscribed to both lists, will they get two copies?

No. The email system has rules and filters in place that prevent someone from receiving two copies of the same email)

An example: Janet is subscribed to two of your email newsletters (List A and List B) and when you send your email you select to send the message to both lists at the same time. Janet would only receive one copy of the message.

More notes on duplicates and removing them from your lists. Information on sending a message could also be helpful.