Do not allow the greeter to send the same password multiple times (Fix GPG smart card access)

Summary:
Greetings,

I am working on improving some of the functionality in the screen locker.  Specifically, I am attempting to fix [this bug](https://lists.gnupg.org/pipermail/gnupg-devel/2019-September/034443.html) reported to the gnupg-devel mailing list.  See also a bug that I reported to the Debian package maintainer: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=934185.  That bug report has not received a response, so I've come here.

Some background: The [libpam-poldi](https://packages.debian.org/buster/libpam-poldi) package for Debian allows for 2FA with GnuPG smart cards into a Linux workstation.  This functionality is very imporant to me and to my organization.  This patch will fix a problem with the conversation between Poldi and KScreenLocker.

To reproduce the bug using libpam-poldi and the screen locker:

1. arrive at the screen locker
2. remove and re-insert the GPG smart card
3. enter a PIN l...