Not sure how it happened, but looks too cool not to share.
Let me guess: originally this came from Sibelius, and the .sib file was affected (infected?) by the “you accidentally made multiple copies of a slur without realizing you did that, because they are all lying exactly on top of each other, so you can’t see they are there” feature. 
Nope - came from Finale.
That’s amazing! Kinda wish I could do that on purpose.
I really wanted to do some kind of visual poetry/art scores in Dorico to see how far one could push it, but, alas, these kinds of projects aren’t very high on one’s priorities, as you can imagine…