468様

ご回答いただきありがとうございます。
ログを確認したところ、

[2020-05-16 20:52:11,836304] eccube.INFO [391042aa] [ffca72dd] [NULL] [LogListener:logResponse:96] - < 200 [GET, /shopping/complete, 121.93.72.220, https://mysite.com/shopping, Mozilla/5.0 (iPhone; CPU iPhone OS 13_4_1 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/13.1 Mobile/15E148 Safari/604.1]
[2020-05-16 20:52:11,830696] eccube.INFO [391042aa] [ffca72dd] [NULL] [RouterListener:onKernelRequest:145] - Matched route "block_recommend_product_block". {"route_parameters":{"_controller":"\\Plugin\\Recommend\\Controller\\Block\\RecommendController::index","_route":"block_recommend_product_block"},"request_uri":"https://mysite.com/block/recommend_product_block"} [GET, /shopping/complete, 121.93.72.220, https://mysite.com/shopping, Mozilla/5.0 (iPhone; CPU iPhone OS 13_4_1 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/13.1 Mobile/15E148 Safari/604.1]
[2020-05-16 20:52:11,718886] eccube.INFO [391042aa] [ffca72dd] [NULL] [RouterListener:onKernelRequest:145] - Matched route "block_search_product". {"route_parameters":{"_controller":"\\Eccube\\Controller\\Block\\SearchProductController::index","_route":"block_search_product"},"request_uri":"https://mysite.com/block/search_product"} [GET, /shopping/complete, 121.93.72.220, https://mysite.com/shopping, Mozilla/5.0 (iPhone; CPU iPhone OS 13_4_1 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/13.1 Mobile/15E148 Safari/604.1]
[2020-05-16 20:52:11,716867] eccube.INFO [391042aa] [ffca72dd] [NULL] [RouterListener:onKernelRequest:145] - Matched route "block_cart". {"route_parameters":{"_controller":"\\Eccube\\Controller\\Block\\CartController::index","_route":"block_cart"},"request_uri":"https://mysite.com/block/cart"} [GET, /shopping/complete, 121.93.72.220, https://mysite.com/shopping, Mozilla/5.0 (iPhone; CPU iPhone OS 13_4_1 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/13.1 Mobile/15E148 Safari/604.1]
[2020-05-16 20:52:11,611449] eccube.INFO [391042aa] [ffca72dd] [NULL] [LogListener:logRequest:83] - > GET /shopping/complete [GET, /shopping/complete, 121.93.72.220, https://mysite.com/shopping, Mozilla/5.0 (iPhone; CPU iPhone OS 13_4_1 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/13.1 Mobile/15E148 Safari/604.1]
[2020-05-16 20:52:11,610794] eccube.INFO [391042aa] [ffca72dd] [NULL] [AnonymousAuthenticationListener:handle:62] - Populated the TokenStorage with an anonymous Token. [GET, /shopping/complete, 121.93.72.220, https://mysite.com/shopping, Mozilla/5.0 (iPhone; CPU iPhone OS 13_4_1 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/13.1 Mobile/15E148 Safari/604.1]
[2020-05-16 20:52:11,610454] eccube.INFO [NULL] [ffca72dd] [NULL] [RouterListener:onKernelRequest:145] - Matched route "shopping_complete". {"route_parameters":{"_controller":"\\Eccube\\Controller\\ShoppingController::complete","_route":"shopping_complete"},"request_uri":"https://mysite.com/shopping/complete"} [GET, /shopping/complete, 121.93.72.220, https://mysite.com/shopping, Mozilla/5.0 (iPhone; CPU iPhone OS 13_4_1 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/13.1 Mobile/15E148 Safari/604.1]
[2020-05-16 20:52:11,408549] eccube.INFO [391042aa] [a13defe9] [NULL] [LogListener:logResponse:94] - < 302 https://mysite.com/shopping/complete [GET, /shopping/epsilon_payment/complete?trans_code=xxxxxxxx&user_id=non_customer&result=1&order_number=4655&duplicate=1, 121.93.72.220, https://mysite.com/shopping, Mozilla/5.0 (iPhone; CPU iPhone OS 13_4_1 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/13.1 Mobile/15E148 Safari/604.1]
[2020-05-16 20:52:11,185312] eccube.INFO [391042aa] [a13defe9] [NULL] [LogListener:logRequest:83] - > GET /shopping/epsilon_payment/complete?trans_code=xxxxxxxx&user_id=non_customer&result=1&order_number=4655&duplicate=1 [GET, /shopping/epsilon_payment/complete?trans_code=xxxxxxxx&user_id=non_customer&result=1&order_number=4655&duplicate=1, 121.93.72.220, https://mysite.com/shopping, Mozilla/5.0 (iPhone; CPU iPhone OS 13_4_1 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/13.1 Mobile/15E148 Safari/604.1]
[2020-05-16 20:52:11,184558] eccube.INFO [391042aa] [a13defe9] [NULL] [AnonymousAuthenticationListener:handle:62] - Populated the TokenStorage with an anonymous Token. [GET, /shopping/epsilon_payment/complete?trans_code=xxxxxxxx&user_id=non_customer&result=1&order_number=4655&duplicate=1, 121.93.72.220, https://mysite.com/shopping, Mozilla/5.0 (iPhone; CPU iPhone OS 13_4_1 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/13.1 Mobile/15E148 Safari/604.1]
[2020-05-16 20:52:11,182718] eccube.INFO [NULL] [a13defe9] [NULL] [RouterListener:onKernelRequest:145] - Matched route "epsilon_shopping_payment_complete". {"route_parameters":{"_controller":"\\Plugin\\GmoEpsilon\\Controller\\PaymentController::complete","_route":"epsilon_shopping_payment_complete"},"request_uri":"https://mysite.com/shopping/epsilon_payment/complete?duplicate=1&order_number=4655&result=1&trans_code=xxxxxxxx&user_id=non_customer"} [GET, /shopping/epsilon_payment/complete?trans_code=xxxxxxxx&user_id=non_customer&result=1&order_number=4655&duplicate=1, 121.93.72.220, https://mysite.com/shopping, Mozilla/5.0 (iPhone; CPU iPhone OS 13_4_1 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/13.1 Mobile/15E148 Safari/604.1]
[2020-05-16 20:52:05,025048]

というログが出ていました。（長文申し訳ありません）
カスタマイズはオーナーズストアからプラグインをいくつか入れていますので、デフォルトというわけではありません。

また、受注番号4655の受注メールのみ再送されており、その他再送されたメールはありませんでした。
状況再現ができないので詳細がわからないかと思いますが、部分的にでもお分かりになられるようでしたらご意見をお聞かせください。よろしくお願いいたします。