Add copy_from attribute to FieldType (?)
This would allow e.g. a "text" field and a case-sensitive "text_cs" field (sans LowercaseFilter on the analyzer), without requiring the user to pass the same string to two fields in add_document() (maybe the code could use itertools.tee).
schema = Schema(text1=TEXT(analyzer=ana1), text2=TEXT(analyzer=ana2, copy_from="text1") }}}
This would be a burden on the backend (every separate implementation of add_document() would need to check for fields with copy_from and duplicate the content), but this might be made easier if they can all call a base class method to do a lot of the work.