Why does the Inbox rate get worse if I test the same message multiple times?

If you don’t have a consistent email flow coming from your domain every day, for example, system notifications, transaction emails, autoresponders, follow-ups, etc., it is not recommended to run multiple tests for the same message within a short time as inbox providers can track this activity and mark your message as spam. It i advised that you run the tests with an interval of 15-20 minutes to receive as equitable results as possible.

Read this article to learn more about the seed test best practices.