How to find duplicate word in string?

a!localVariables(
  local!string: "Big black bug bit a big black dog on his big black nose",
  local!array: split(local!string," "),
  local!uniqueItems: union(local!array,local!array),
  a!forEach(
    items: local!uniqueItems,
    expression: if(
      count(wherecontains(fv!item,local!array))>1,
      fv!item,
      {}
    )
  )
)

Handling Unnecessary spaces:

a!localVariables(
local!string: "Big black bug bit a big black dog on his big black nose nose ",
local!data: split(local!string, " "),
/* Handling null in case of unnecessary spaces */
local!list: reject(fn!isnull, local!data),
local!result: a!forEach(
items: local!list,
expression: if(
length(wherecontains(fv!item, local!list)) > 1,
fv!item,
{}
)
),
union(local!result, local!result)
)