Think this is a bug:
Have a file with this content
test 1
test 2
test 3
test 4
select test
on the line test 3
ctrl + d (select 3 & 4)
ctrl + d (select 3 & 4 & 1)
ctrl + k, ctrl + d ( try skip 1, which just selected)
actually results in 1 & 2 & 3 selected