[{_id:1,sections:[{Title:"hello world",Title2:"",otherField:"test"},{Title:"goodbye",Title2:""}]},{_id:2},]

db.collection.update({sections:{$exists:true}},[{$addFields:{"sections":{$map:{input:"$sections",as:"s",in:{$mergeObjects:["$$s",{"Title2":"$$s.Title"}]}}}}}],{multi:true})/** "sections.Title2": "$sections.Title",*/