--repair in mongodump doesn't do the same thing as --repair on the server and the language at http://docs.mongodb.org/manual/reference/program/mongodump/ is incorrect.

Description should be clear that --repair is read-only and:

a) for MMAPv1 does a forward and backward pass, iterating through documents, resulting in duplication of each document if the database is in a valid state, and
b) doesn't do anything (as yet) on other storage engines.

--repair
Runs a repair option in addition to dumping the database. The repair option attempts to repair a database that may be in an invalid state as a result of an improper shutdown or mongod crash.
The --repair option uses aggressive data-recovery algorithms that may produce a large amount of duplication.