Reduce the possibility for accidental bk_query duplication by adding a new yaml field
In general a user will not need to use a given bk_query twice in a single production version, there are however possible legitimate use cases. Currently a user will just receive a warning if they duplicate a bk_query but it would be better to fail yaml validation when a duplicate bk_query is detected unless the user makes it clear they intend to do this by use of a new yaml field boolean allow_duplicate_bk_query.
As discussed in !35 (merged) concerning the lhcb-datapkg/AnalysisProductions!151 (merged) production which does need to duplicate bk_query entries but was being blocked by the restriction on duplicates.