Back in the 70's the Legion of Superheroes experimented with clones. The icky part was they were exact duplicates of members who had perished. Nor were they told they were clones. The Legion tested them by sending them back to Superboy's time where they proved they were as brave as the originals were. After doing so they exploded. Apparently clones explode thus preventing the Legion from really using the clone technology till they fixed that problem.
