Fixed race condtion while importing history types for the first time.

lib/import/otrs/history.rb

@@ -1,9 +1,11 @@
+# rubocop:disable Style/ClassVars
 module Import
   module OTRS
     class History
 
       def initialize(history)
         init_callback(history)
+        ensure_history_attribute
         add
       end
 
@@ -16,6 +18,18 @@ module Import
       def add
         ::History.add(@history_attributes)
       end
+
+      # make sure that no other thread is importing just the same
+      # history attribute which causes a ActiveRecord::RecordNotUnique
+      # exception we (currently) can't handle otherwise
+      def ensure_history_attribute
+        history_attribute = @history_attributes[:history_attribute]
+        return if !history_attribute
+        @@created_history_attributes ||= {}
+        return if @@created_history_attributes[history_attribute]
+        @@created_history_attributes[history_attribute] = true
+        ::History.attribute_lookup(history_attribute)
+      end
     end
   end
 end