The single-celled organism Tetrahymena has two cell nuclei. One contains the working copy of its DNA database.
The other contains a replicated slave DB, which acts as a checksum to the original in case it becomes corrupted.
The BBC has the story.
"The smaller nucleus (called the micronucleus) does nothing more than keep the cell's full genome safe. It seems that Tetrahymena uses the smaller of its two nuclei as a master record of its dna so that it always has a safe set of genes for the cell's offspring.
"The other nucleus, called the macronucleus, uses 'working' dna to regulate the cell's life functions."