duplicate rows returned from SELECT where KEY term is duplicated

Noticed while working on CASSANDRA-2268 when random keys generated during a mutli_get test contain duplicate keys.

The thrift multiget_slice() returns only the unique rows because of the map generated for the result.

CQL will return a row for each KEY term in the SELECT.

I could make QueryProcessor.getSlice() only create commands for the unique keys if we wanted to.

Not sure it's a bug and it's definitely not something that should come up to often, reporting it because it's different to the thrift mutli_get operation.

Happy to close if it's by design.