How do you delete Reminders from your iphone?

I own an iPhone 6. I have not had it jail broken and the phone has not suffered any kind of damage. I also do not use iCloud storage for reminders.

I have a long list of old completed reminders that I deleted but have remained on my completed list.

I try to delete them by tapping edit and then tapping the red minus icon and then tapping delete.

But it will either ignore me tapping delete and remain on the list, or it will act like it was deleted and go a way, when in reality it just moved to the bottom of the list and was duplicated so now I have two or more of the same reminder taking up space.

How do I permanently remove reminders and stop them from duplicating themselves when I try to delete them?

Individually swipe them left and tap on the red 'delete'.