In the Qur’an Muhammad and Ahmad is the same person with two roles –
Muhammad is the last and the seal of the prophet which also mean the Qur’an is the final book from Allah –
S33:40 Muhammad is not the father of any of your men, but he is the Messenger of Allah and the Seal of the Prophets: and Allah has full knowledge of all things.
Allah send His scriptures through the prophets –
S2:213 Mankind was of one single nation then Allah sent the prophets as bringers of good tidings and warners and sent down with them the Scripture in truth to judge between the people concerning that in which they differed…….
Ahmad, in the Qur’an is identified as the Messenger of Allah –
S61:6 And remember, Jesus, the son of Mary, said: “O Children of Israel! I am the messenger of Allah sent to you, confirming the Law which came before me, and giving Glad Tidings of a Messenger to come after me, whose name shall be Ahmad.” But when he came to them with Clear Signs, they said, “this is evident sorcery!”
S33:20 Ye have indeed in the Messenger of Allah a beautiful pattern of conduct for any one whose hope is in Allah and the Final Day, and who engages much in the Praise of Allah.
During genesis the chosen prophets were made to make a covenant –
S3:81 Behold! Allah took the covenant of the prophets, saying: “I give you a Book and Wisdom; then comes to you a messenger, confirming what is with you; do ye believe in him and render him help.” Allah said: “Do ye agree, and take this my Covenant as binding on you?” They said: “We agree.” He said: “Then bear witness, and I am with you among the witnesses.”
All the prophets agreed to believe in the messenger and render him help when the Messenger come to them to confirm what is with them. However, during the time of Prophet Muhammad who is the last prophet, the only name that was mentioned as the messenger of Allah is Ahmad.
So Ahmad and Muhammad is the same person as they were mentioned to be the messenger who confirmed the scripture the Qur’an which the prophet believe and to render him help respectively.
S7:158 Say: “O men! I am sent unto you all, as the Messenger of Allah, to Whom belong the dominion of the heavens and the earth: there is no god but He: it is He That give both life and death. So believe in Allah and His Messenger, the Unlettered Prophet, who believe in Allah and His words: follow him that so ye may be guided.”
Since all the prophets are now dead anyone who claimed to be the messenger of Allah is an imposter as there is no new scripture after the Qur’an that will come for him to confirm and there is no more prophet to render him help as all are dead.
Furthermore, Muhammad the last prophet does not need other messenger to confirm his message as he had been confirmed by Ahmad the messenger of Allah.
Those who claimed to be the messenger of Allah after Muhammad are making fabrication which contradicted with the message of the Qur’an.
S6:93 Who can be more wicked than one who invent a lie against Allah, or saith, “I have received inspiration,” when he hath received none, or who saith, “I can reveal the like of what Allah hath revealed”? If thou could but see how the wicked do fare in the flood of confusion at death! the angels stretch forth their hands, (saying),”Yield up your souls: this day shall ye receive your reward, a penalty of shame, for that ye used to tell lies against Allah, and scornfully to reject of His signs!
Any community that have not been visited by a messenger, a warner will come to give warning –
S25:1 Blessed is He who sent down the Criterion upon His Servant that he may be to the worlds a warner.
S34:34 Never did We send a warner to a population, but the wealthy ones among them said: “We believe not in the Message with which ye have been sent.”
S67:8-10 It almost bursts with rage. Every time a company is thrown into it, its keepers ask them, “Did there not come to you a warner?” They will say,” Yes, a warner had come to us, but we denied and said, ‘Allah has not sent down anything. You are not but in great error.'” And they will say, “If only we had been listening or reasoning, we would not be among the companions of the Blaze.”
S6:155 And this [Qur’an] is a Book We have revealed which is blessed, so follow it and fear Allah that you may receive mercy.
Pondering with Saifuddin's index 