[jxf]

That definitely wouldn't work with this strategy (two copies), because you wouldn't be able to tell which copy was the mutated one. Error-correcting codes to the rescue. :)

[ojii]

couldn't one make 3 copies and take the two that are equal as non-modified?

[eklavya]

So like above said, error correction for at most one error.

[Guillaume86]

2 copy + one hash could do the same job?