Must be a glitch with the KFC app.
I've collected 11 stamps, which entitles me to £5 off.
I used the £5 off, and the app still showed I had 11 stamps.
Used again and straight away, £5 off again!
The app is still showing I'm entitled to another £5 off.
So far, so good! I'm saving £5 on every visit to KFC!