Shared sheets can create issues in a complex hierarchy.
This is not related to sharing sheets between projects.
(Sharing sheets between projects is not supported by Kicad)
ADDED: Back annotation algorithm,
eeschema back annotation dialog
CHANGED: added some minor helper methods to SCH_REFERENCE_LIST and SCH_REFERENCE,
split SCH_REFERENCE_LIST::CheckAnnotation on 2 parts to reuse code