Full_Name: Howard Chu
Submission from: (NULL) (220.127.116.11)
Submitted by: hyc
When the next value to return is a duplicate value, cursor_get(MDB_NEXT_DUP)
skips returning the key, because it is assumed to already be known.
cursor_get(MDB_NEXT) also skips returning the key because it falls thru the same
code, but in this case we cannot assume the key is already known.