Calander alerts

If running iOS 13 or earlier versions, check: Settings - Passwords & Accounts - Accounts - Any rogue entries here? If so, delete the rogue account. 

If running iOS 14, check: Settings - Calendar - Accounts - Any rogue entries here? If so, delete the rogue account. 

The Apple support article is below. 

https://support.apple.com/en-us/HT211076

So you can go and disable them or else just try to restart you phone and check it, Earlier I was also having that problem I deleted this form calendar and restarted my iPhone and the problem was fixed.