on old versions
this script:
:do {/ip firewall address-list add address=22.22.22.22 comment="Banned-X" list=list-X} on-error={ :put "failed add 22.22.22.22"};
on subsequent executions, return on-error={ :put “failed add 22.22.22.22”}
after version v6.36
this script:
:do {/ip firewall address-list add address=google.com comment="google.com" timeout=0 list="BAN-list"} on-error={:put "failed add google.com, already exists."};
does not return on-error, adds a duplicate (this bug only with the URL, when adding IP - returns on-error)
PS: when added through WinBox the same problem - it is possible to create a duplicate