Fixes #3090: Failed to run import backend 'Import::Exchange'. Cause: Missing implementation of 'mapping' method for 'Sequencer::Unit::Import::Exchange::FolderContact::Mapping::Login'.

lib/sequencer/unit/import/common/mapping/mixin/provide_mapped.rb

@@ -14,7 +14,11 @@ class Sequencer
               private
 
               def existing_mapped
-                @existing_mapped ||= mapped || ActiveSupport::HashWithIndifferentAccess.new
+                @existing_mapped ||= begin
+                  # we need to use `state.optional` instead of just `mapped` here
+                  # to prevent naming conflicts with other Unit methods named `mapped`
+                  state.optional(:mapped) || ActiveSupport::HashWithIndifferentAccess.new
+                end
               end
 
               def provide_mapped