Steps to reproduce:

Try to upgrade zabbix-release from 4.0 to 4.2

Result:

dpkg: Warnung: Version 1:4.0-2+stretch des Paketes zabbix-release wird durch ältere Version 4.2-1+stretch ersetzt

Expected:

Installation without complains.

The Version Number is wrong: 4.0 has

ii  zabbix-release                       1:4.0-2+stretch                all          Zabbix official repository configuration

but 4.2 has

ii  zabbix-release                       4.2-1+stretch                  all          Zabbix official repository configuration

so dpkg is taking this as a downgrade. Please fix the version number, thank you