If life on Mars had a separate origin (say, with a different genetic code) it would be strong evidence that life is common in the universe.
However, they'd need to rule out transfer of life between Mars and the early Earth. If they can't do that, this is still somewhat interesting, since it could mean life was transferred back and forth between Mars and Earth during that period. It might also mean there's still life deep in Mars' crust where liquid water can still exist, and that life could give more information on what early life was like (being another branch of our evolutionary tree).
However, they'd need to rule out transfer of life between Mars and the early Earth. If they can't do that, this is still somewhat interesting, since it could mean life was transferred back and forth between Mars and Earth during that period. It might also mean there's still life deep in Mars' crust where liquid water can still exist, and that life could give more information on what early life was like (being another branch of our evolutionary tree).