Thomas Riker was created through a complete freak accident involving a distortion field that reflected a transporter beam in the exact right way to duplicate Will Riker. It's not something that be done easily at will.

Now cloning and memory implantation are completely possible as well, like that one TNG episode where they make a clone of Kahless or that one very dumb ENT episode where there's a clone of Trip. I'm not sure, but cloning is probably illegal by the time of Voyager, since genetic augmentation is completely illegal throughout the Federation.

But the main problem is that nothing other than killing Tuvix would have satiated Janeway's bloodlust. For real, she's like the most evil person in that entire show.