Did you test it?
```
irb(main):006:0> interfaces='["eth0", "eth0"]'
=> "[\"eth0\", \"eth0\"]"
irb(main):007:0> parsed_ifs = JSON.parse(interfaces)
=> ["eth0", "eth0"]
irb(main):008:0> cmd = "dhcpcheck #{parsed_ifs}"
=> "dhcpcheck eth0eth0"
```