If carefully considered, the comma isn't mistakenly added. It's perfect where it is. There is a printer's error, or editorial error of a missing comma that was missed in editorial reprinting. Compare all the other verses that use similar languge as you noted previously. If you want I can probably post that for you:
And he saith unto him,
Verily, verily, I say unto you, Hereafter ye shall see heaven open, and the angels of God ascending and descending upon the Son of man. -
John 1:51
Jesus answered and said unto him,
Verily, verily, I say unto thee, Except a man be born again, he cannot see the kingdom of God. -
John 3:3
Jesus answered,
Verily, verily, I say unto thee, Except a man be born of water and [of] the Spirit, he cannot enter into the kingdom of God. -
John 3:5
Verily, verily, I say unto thee, We speak that we do know, and testify that we have seen; and ye receive not our witness. -
John 3:11
Then answered Jesus and said unto them,
Verily, verily, I say unto you, The Son can do nothing of himself, but what he seeth the Father do: for what things soever he doeth, these also doeth the Son likewise. -
John 5:19
Verily, verily, I say unto you, He that heareth my word, and believeth on him that sent me, hath everlasting life, and shall not come into condemnation; but is passed from death unto life. -
John 5:24
Verily, verily, I say unto you, The hour is coming, and now is, when the dead shall hear the voice of the Son of God: and they that hear shall live. -
John 5:25
Jesus answered them and said,
Verily, verily, I say unto you, Ye seek me, not because ye saw the miracles, but because ye did eat of the loaves, and were filled. -
John 6:26
Then Jesus said unto them,
Verily, verily, I say unto you, Moses gave you not that bread from heaven; but my Father giveth you the true bread from heaven. -
John 6:32
Verily, verily, I say unto you, He that believeth on me hath everlasting life. -
John 6:47
Then Jesus said unto them,
Verily, verily, I say unto you, Except ye eat the flesh of the Son of man, and drink his blood, ye have no life in you. -
John 6:53
Jesus answered them,
Verily, verily, I say unto you, Whosoever committeth sin is the servant of sin. -
John 8:34
Verily, verily, I say unto you, If a man keep my saying, he shall never see death. -
John 8:51
Jesus said unto them,
Verily, verily, I say unto you, Before Abraham was, I am. -
John 8:58
Verily, verily, I say unto you, He that entereth not by the door into the sheepfold, but climbeth up some other way, the same is a thief and a robber. -
John 10:1
Then said Jesus unto them again,
Verily, verily, I say unto you, I am the door of the sheep. -
John 10:7
Verily, verily, I say unto you, Except a corn of wheat fall into the ground and die, it abideth alone: but if it die, it bringeth forth much fruit. -
John 12:24
Verily, verily, I say unto you, The servant is not greater than his lord; neither he that is sent greater than he that sent him. -
John 13:16
Verily, verily, I say unto you, He that receiveth whomsoever I send receiveth me; and he that receiveth me receiveth him that sent me. -
John 13:20
When Jesus had thus said, he was troubled in spirit, and testified, and said,
Verily, verily, I say unto you, that one of you shall betray me. -
John 13:21
Jesus answered him, Wilt thou lay down thy life for my sake?
Verily, verily, I say unto thee, The cock shall not crow, till thou hast denied me thrice. -
John 13:38
Verily, verily, I say unto you, He that believeth on me, the works that I do shall he do also; and greater [works] than these shall he do; because I go unto my Father. -
John 14:12
Verily, verily, I say unto you, That ye shall weep and lament, but the world shall rejoice: and ye shall be sorrowful, but your sorrow shall be turned into joy. -
John 16:20
And in that day ye shall ask me nothing.
Verily, verily, I say unto you, Whatsoever ye shall ask the Father in my name, he will give [it] you. -
John 16:23
Verily, verily, I say unto thee, When thou wast young, thou girdedst thyself, and walkedst whither thou wouldest: but when thou shalt be old, thou shalt stretch forth thy hands, and another shall gird thee, and carry [thee] whither thou wouldest not. -
John 21:18
For verily I say unto you, Till heaven and earth pass, one jot or one tittle shall in no wise pass from the law, till all be fulfilled. -
Matthew 5:18
Verily I say unto thee, Thou shalt by no means come out thence, till thou hast paid the uttermost farthing. -
Matthew 5:26
Therefore when thou doest [thine] alms, do not sound a trumpet before thee, as the hypocrites do in the synagogues and in the streets, that they may have glory of men.
Verily I say unto you, They have their reward. -
Matthew 6:2
And when thou prayest, thou shalt not be as the hypocrites [are]: for they love to pray standing in the synagogues and in the corners of the streets, that they may be seen of men.
Verily I say unto you, They have their reward. -
Matthew 6:5
Moreover when ye fast, be not, as the hypocrites, of a sad countenance: for they disfigure their faces, that they may appear unto men to fast.
Verily I say unto you, They have their reward. -
Matthew 6:16
When Jesus heard [it], he marvelled, and said to them that followed,
Verily I say unto you, I have not found so great faith, no, not in Israel. -
Matthew 8:10
Verily I say unto you, It shall be more tolerable for the land of Sodom and Gomorrha in the day of judgment, than for that city. -
Matthew 10:15
But when they persecute you in this city, flee ye into another: for
verily I say unto you, Ye shall not have gone over the cities of Israel, till the Son of man be come. -
Matthew 10:2
And whosoever shall give to drink unto one of these little ones a cup of cold [water] only in the name of a disciple,
verily I say unto you, he shall in no wise lose his reward. -
Matthew 10:42
For verily I say unto you, That many prophets and righteous [men] have desired to see [those things] which ye see, and have not seen [them]; and to hear [those things] which ye hear, and have not heard [them]. -
Matthew 13:17