Prevent adding duplicate entries

[Steve]

Currently, it is possible to scan a QR code twice and add duplicate entries. The codes are exactly the same. The app should prevent this.

[Bruno Bierbaumer]

Thanks for reporting this! I will fix this when I am at the code again :)

[mueller-ma]

This repo seems to be unmaintained, but there is a active fork, which is also in F-Droid: https://github.com/kckrinke/onetimepad