bolt: duplicated quads are not skipped properly

[dennwc]

Steps to reproduce the issue:

cayley init -d bolt -a ./test.db
cayley load -d bolt -a ./test.db -i ./data/testdata.nq
cayley load -d bolt -a ./test.db -i ./data/testdata.nq
cayley dump -d bolt -a ./test.db -o=-

Received results:

<alice> <follows> <bob> .
<bob> <follows> <fred> .
<bob> <status> "cool_person" .
...
<alice> <follows> <bob> .
<bob> <follows> <fred> .
<bob> <status> "cool_person" .
...

Note duplicated quads.

Expected results:

<alice> <follows> <bob> .
<bob> <follows> <fred> .
<bob> <status> "cool_person" .
...

Output of cayley version or commit hash:

Cayley version: 0.7.0
Git commit hash: 6f8446e4171ebc84955dd0ff91927510ba1aee16
Build date: 2017-12-24T17:51:48Z

[eshijia]

@dennwc Recently, I have found that if a data file itself contains duplicate quads, the latest version of Cayley will still retain the duplicated quads after the cayley load command.