Can cause problems when covered indexes are used, and likely in other scenarios.

> c.drop()
true
> c.save( {a:[0.0,NumberLong(0)]} )
> c.find( {a:0}, {a:1,_id:0} )
{ "a" : [ 0, NumberLong(0) ] }
> c.ensureIndex( {a:1} )
> c.find( {a:0}, {a:1,_id:0} )
{ "a" : 0 }

Probably doesn't make sense to allow multiple equal keys per doc in the short term, since without extra work that would break uniqueness of key,doc pairs which would would be bad for the btrees and likely other places. But we can try prevent bad results as in above.