this is a difference in behavior from the legacy tools.

classic behavior:
"mongorestore --w=1" will keep going if it encounters a duplicate key error.

new behavior:
"mongorestore --w=1" will short circuit and fail if it encounters a duplicate key error.

cc dan@10gen.com
which behavior is better?

1. continue inserting on duplicate key errors
2. short circuit and fail on a duplicate key error
3. have a --stopOnError flag which controls this behavior (which is what mongoimport does)