[{"_id":ObjectId("61968a26c05149a23ad391f4"),"letters":["aa","ab","ac","ad","aa","af"],"numbers":[11,12,13,14]},{"_id":ObjectId("61968a26c05149a23ad391f5"),"letters":["ab","af","ag","ah","ai","aj"],"numbers":[15,16,17,18]},{"_id":ObjectId("61968a26c05149a23ad391f6"),"letters":["ac","ad","ae","af","ag","ah"],"numbers":[16,17,18,19]},{"_id":ObjectId("61968a26c05149a23ad391f7"),"letters":["ae","af","ag","ah","ai","aj"],"numbers":[17,18,19,20]}]

db.collection.aggregate([{$unwind:"$letters"},{$group:{_id:null,all_letters:{"$addToSet":"$letters"}}},{$merge:{into:"dst_coll"}}])