#APL 29#


This is a nice question for the key operator (⌸) which was introduced with Dyalog APL v14.
It takes the left argument function ({1↑1↓⍵}) and gives it for each unique argument, the indices in the vector for that argument.
Here I'm taking the second index, then i check which of the indices is present in this list ((⍳⍴⍵)∊) and use the resulting boolean for splitting the original vector.

Can be tried online here:


  [1]: http://tryapl.org/