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I couldn't find any help about this topic, but if I have a campaign made for clients of my product X, and I have another campaign for clients of my product Y, can I send one mail to those two lists to avoid duplicate mails?
When you say, "product X & Y", do you mean "brands" in Sendy? Or "lists"?
If you meant "brands", no you can't send the same campaign to lists in both brands hoping emails won't be sent to recipients that may exist in both brands.
If you meant "lists", a recipient that appears in 2 lists on the same brand won't receive the same email twice if you send a campaign to both lists.