Remove duplicates from node list

[ryotayamanaka]

Test case:

http://localhost:7000/merge_node/?label=person&id=Taro&props={"age":[37]}
http://localhost:7000/merge_node/?label=person&id=Jiro&props={"age":[36]}
http://localhost:7000/merge_edge/?label=knows&src_id=Taro&dst_id=Jiro&props={"since":[2017]}
http://localhost:7000/merge_edge/?label=knows&src_id=Jiro&dst_id=Taro&props={"since":[2017]}

curl http://localhost:7000/edge_match/?labels=knows