ensureIndex() fails to create unique index if non-unique index already exists

db.foo.ensureIndex(

{a : 1}

);
db.foo.save(

{key:1, a:1}

);
db.foo.save(

{key:2, a:2}

);

db.foo.ensureIndex(

{a : 1}

,

{name:'uniquea', unique:true}

);

db.foo.getIndexes() shows that 'uniquea' has not been created. While we should be checking to avoid creating exact duplicate indexes, my attempt to create a new unique index should succeed, because there isn't already a unique one. I might be doing this in preparation for replacing the (non-unique) 'a_1' index with 'uniquea'.