Fix duplicate cell_ids and milestone_ids

Eg. prefix duplications with an M

Wishbone also affected by this

@rcannood I don't remember, is it now possible to remove this requirement?

Line 45 in 3b55c60

testthat::expect_false(any(duplicated(c(milestone_ids, cell_ids))))

Only the geodesic distances were affected by this, right?

I will remove this requirement then 😇

Thanks @zouter !