Look at the context, for once in your life!
Rom. 5:14-19 (KJV)
14 Nevertheless death reigned from Adam to Moses, even over them that had not sinned after the similitude of Adam’s transgression, who is the figure of him that was to come.
15 But not as the offence, so also is the free gift. For if through the offence of one many be dead, much more the grace of God, and the gift by grace, which is by one man, Jesus Christ, hath abounded unto many.
16 And not as it was by one that sinned, so is the gift: for the judgment was by one to condemnation, but the free gift is of many offences unto justification.
17 For if by one man’s offence death reigned by one; much more they which receive abundance of grace and of the gift of righteousness shall reign in life by one, Jesus Christ.)
18 Therefore as by the offence of one judgment came upon all men to condemnation; even so by the righteousness of one the free gift came upon all men unto justification of life.
19 For as by one man’s disobedience many were made sinners, so by the obedience of one shall many be made righteous.
This whole passage is about the contrast between the first Adam and the last Adam; it is also about the contrast between those who are in the first Adam and those who are in the last Adam. This could hardly be clearer.
If the two "all men" groups, in verse 18, were one and the same, then the whole context of the passage would be broken; not only that, but also it would mean that every single person who ever lived would receive justification unto life! I hope you realise that this is heretical...