LineOffset is crashing with two identical coordinates in sequence

I'm getting the error:

catch, Error: coordinates must contain numbers

when there are two or more identical coordinates in sequence:

          const arr1 = [[1,1], [1,1]]
          const x = lineString(arr1)
          const y = lineOffset(x, 0.001)

but this works:

          const arr1 = [[1,1],[12,12], [1,1]]
          const x = lineString(arr1)
          const y = lineOffset(x, 0.001)

It isn't mentioned in the docs that it's not ok to have identical points...

Start by reproducing the issue through the lineString and lineOffset entry points with consecutive identical coordinates, then compare it with the working non-consecutive example. Done means lineOffset no longer crashes for the reported input, with behavior for duplicate points clarified or covered by a regression test.