If Starfleet had the ability to duplicate that freak transporter accident at will, do you honestly think Section 31 wouldn't do that to their agents before sending them on a dangerous mission and put the original in stasis until they knew whether the duplicate had died or not?

Also, someone's already asked that here. If you duplicated Tuvix, you have exactly the same ethical dilemma as to whether to murder the new Tuvix (I can't in good conscience call that one the Twovix thanks to the brilliant LD episode, but maybe I could get away grammatically with Toovix?).

In fact, that would be even worse. "Sorry guy, we created you in a transporter specifically so we could murder you and bring back the originals...". At least the original Tuvix was an accident. Creating a sentient being specifically to murder it is... well, it's something Archer would do, come to think about it. RIP Sim.