duplicate rows in view results when mapping doc id to null

Looks like regression, although I maybe did something I didn't try earlier.

Managed to hit this:

1. wget O 'http://lh:9503/default/_design/t/_view/t?stale=false&connection_timeout=60000&limit=10&skip=0'
   -2012-10-31 12:48:55- http://lh:9503/default/_design/t/_view/t?stale=false&connection_timeout=60000&limit=10&skip=0
   Resolving lh (lh)... 127.0.0.1
   Connecting to lh (lh)|127.0.0.1|:9503... connected.
   HTTP request sent, awaiting response... 200 OK
   Length: unspecified [text/plain]
   Saving to: 'STDOUT'

[<=> ] 0 --.-K/s {"total_rows":1323824,"rows":[

{"id":"00000000","key":"00000000","value":null}

,

{"id":"00000001","key":"00000001","value":null}

,

{"id":"00000002","key":"00000002","value":null}

,

{"id":"00000003","key":"00000003","value":null}

,

{"id":"00000004","key":"00000004","value":null}

,

{"id":"00000004","key":"00000004","value":null}

,

{"id":"00000005","key":"00000005","value":null}

,

{"id":"00000005","key":"00000005","value":null}

,

{"id":"00000006","key":"00000006","value":null}

,

{"id":"00000007","key":"00000007","value":null}

]
}
[ <=> ] 536 --.-K/s in 0s

2012-10-31 12:48:55 (2.22 MB/s) - written to stdout [536]