2 scientists conducted a study over 20 years where they cloned generations of mice to see what would happen; the researchers created a total of 58 generations before the project ended

By generation 57, the cloned mice had acquired over 3400 single-base changes relative to the starting mouse, whereas 62 generations of natural reproduction (in an inbred mouse strain) had accumulated 752.

In other words, cloning caused 3.1 times more single-nucleotide mutations per generation than natural reproduction.