Hello Natalie,
Thank you for reaching out!
Based on your post, it sounds like you have already covered some of the standard troubleshooting options that I would recommend (deleting/reinstalling the app, managing your notifications, etc.). It is also possible that you may have multiple tokens for accessing the Canvas iOS App which may be causing duplicate notifications. You can actually manage API access tokens from within your User Settings and remove duplicate tokens by referencing the following step-by-step documentation: How to manage API access tokens as a student?
If this does not resolve the issue, I recommend that you create a technical case using the "Report a Problem" button in your Canvas Help Menu so that a member of our Canvas Support team may assist with next steps. I hope this helps!