You have the ability to target Repeat buyers based on their purchase count.

*The purchase count is based on orders, not items purchased.

Inside editing your campaign you can scroll down to "Advanced Options"

Click (show) to see the Buyer's Purchase Count
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The purchase count per buyer is calculated based on the historic order data that is imported during your initial MWS sync.

If you want to send a unique email for "Repeat Buyers" than you would check the 2nd, 3rd, and 4th purchase or more boxes as seen in the screenshot above.
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To learn more about Buyer's Purchase count, please refer to this article.