Test Emails will not send if you have not created a subject line for your email. You must also have created a basic Email design before you begin testing.
If a test fails on more than one occasion, these tests will resend once you correct the issue that was causing the failure. This can result in multiple tests coming through, especially if you send more tests in the meantime.
The best way to avoid this is to review all settings within Manage Content before issuing tests.