Only a sample of your genome is being tested at any of the testing companies, so it is basically impossible for anyone to "hack in" and make a clone out of you.
Here are the numbers I have:
number of SNPs in the human genome: 112,736,879
number of SNPs in a DNA test: 577,382at 23&me, 708,092 at FTDNA, and 682,549 at Ancestry.
Sources:
http://en.wikipedia.org/wiki/Single-nucleotide_polymorphismhttp://www.isogg.org/wiki/Autosomal_DNA_testing_comparison_c...--Elizabeth