I requested to join one time. I was declined an hour later due to multiple requests from my IP address. I re-sent my request. I also replied to the email with the correct handle just in case it happened again.\\What can cause this to happen so I can correct it and avoid this problem in the future?
Usually that multiple requests is used when we have 2 or more accounts ready to be accepted at the same time that use the same IP. We have no idea which is the correct account (as you are only allowed one) so we reject both. If you and another person in your family are trying to join, wait until the first account is accepted before making another attempt to join.
If its just because during your application process you had a refresh issue, or otherwise restarted your application, its possible the first attempt went through - you should wait a litle bit and see if you got an email.
If you keep getting rejected for this issue, put something in the multi field during registration (such as: multi issue, please check) so that the regular account accepters ignore it and leave it for ASims to check the multiple accounts. In this case you should also make a support ticket (Contact Support link at the very bottom of the sidebar) as the emails are temperemental in how they are read/received.