Database
[{"_id":ObjectId("5d654ed25616a9d461bc0aa7"),"aC":"AB","aI":"ABCD","aN":"000000002001","aT":"CA","bID":NumberLong(0),"pD":"2019-06-18","transactions":[{"seqNo":1,"tC":"S"},{"seqNo":2,"tC":"S"},{"seqNo":2,"tC":"S"},{"seqNo":1,"tC":"S"},{"seqNo":3,"tC":"S"}]}]
Query
db.collection.update({"transactions":{"$type":"array"}},[{"$set":{"transactions":{"$setUnion":"$transactions"}}}],{"multi":true})