A duplicate entry in the import will cause RouterOS to stop and not process further down the list (v6.16)
ie (dshield.rsc below)
/ip firewall address-list
add list=DShield address=122.226.73.131/32 timeout=2h
add list=DShield address=85.13.160.11/32 timeout=2h
add list=DShield address=50.23.113.146/32 timeout=2h
add list=DShield address=50.23.113.146/32 timeout=2h
add list=DShield address=122.225.109.212/32 timeout=2h
add list=DShield address=176.31.251.200/32 timeout=2h
add list=DShield address=218.2.0.137/32 timeout=2h
add list=DShield address=122.226.73.131/32 timeout=2h
It will import 50.23.113.146 and stop processing the import. Bug in RouterOS?
Using /import file-name=dshield.rsc; to import list