In Star Trek, people are beamed from place to place using transporters; their molecules are converted into energy, which is then beamed to another place, where it is turned back into the original people. The problem with that is their being converted into energy: as though they were set on fire, or vaporized by a laser beam, or ionized, and then some. [Chaospet Cartoon]
As for that energy turning back into the original people, why should an exact copy of someone be that person? To see why it would not, recall how when people eat they replace some of their atoms with new atoms from food, crapping out some of their old atoms along with some of the unused food. Imagine all of their atoms being replaced with new ones, somehow, with all of the old ones being crapped out. Were that crap collected, and its atoms rearranged into exact copies of those people, surely those copies would be just that: copies. Suppose now that those people do not get new atoms; all that happens now is that the original people are turned into crap, which goes down some tubes, and is then rearranged back into the original forms: much as happens on Star Trek when people are beamed somewhere.