Group

=x: group like values' indexes into sub-lists, in the order their first value is encountered.

  =!10    / no duplicates, a bunch of 1-element lists
(,0
 ,1
 ,2
 ,3
 ,4
 ,5
 ,6
 ,7
 ,8
 ,9)
  =&1 2 3 4 5            / 1x0, 2x1, 3x2, 4x3, 5x4
(,0
 1 2
 3 4 5
 6 7 8 9
 10 11 12 13 14)
  =1 3 5 3 7 5 3 3 8 7 9  / 1x1, 4x3, 2x5, 2x7, 1x8, 1x9
(,0
 1 3 6 7
 2 5
 4 9
 ,8
 ,10)