Issue caused by: https://github.com/vyos/vyos-1x/pull/2879
To reproduce use this on 1.3.x before upgrading to 1.4.x
```
set service dhcp-server shared-network-name LAN authoritative
set service dhcp-server shared-network-name LAN subnet 10.10.10.0/24 default-router '10.10.10.100'
set service dhcp-server shared-network-name LAN subnet 10.10.10.0/24 lease '86400'
set service dhcp-server shared-network-name LAN subnet 10.10.10.0/24 name-server '10.10.10.99'
set service dhcp-server shared-network-name LAN subnet 10.10.10.0/24 range 0 start '10.10.10.1'
set service dhcp-server shared-network-name LAN subnet 10.10.10.0/24 range 0 stop '10.10.10.254'
set service dhcp-server shared-network-name LAN subnet 10.10.10.0/24 static-mapping 002 ip-address '10.10.10.2'
set service dhcp-server shared-network-name LAN subnet 10.10.10.0/24 static-mapping 002 mac-address '7c:d1:c3:75:af:66'
set service dhcp-server shared-network-name LAN subnet 10.10.10.0/24 static-mapping 003 ip-address '10.10.10.2'
set service dhcp-server shared-network-name LAN subnet 10.10.10.0/24 static-mapping 003 mac-address '40:6C:8F:41:B7:78'
```
Migration will fail as there is a duplicate mapping for 10.10.10.2