Basically, there is parsing of the options in the ensureIndex commands but some edge cases need to be ironed out.

dropDups get invoked if ensureIndex options are in array syntax [x=true,y=true]

For example if one inserts

db.test.insert({lname:'David', age:80})
db.test.insert({lname:'David', age:80})
db.test.insert({lname:'David', age:80})

and then issues the command

db.test.ensureIndex({lname:1,age:1},[unique=true,sparse=true])

This will drop all of their duplicate data as entered above.

Further, given the parsing of the options as "object" it doesn't matter what 'key' is passed.

db.test.ensureIndex({lname:1,age:1},[x=true,y=true])

will yield the same results

However

db.test.ensureIndex({lname:1,age:1},[x=false,y=true]) 

shall yield a more peculiar result - the dropDups will be stored system.indexes but unique won't be enforced.

There may be other ways to pass in these options that can be dangerous in similar ways.