I have data in CDT where i need to apply some validations to that data

What kind of validations are you trying to add - just the fact that there are duplicate values or do you need to specify directly which ones are the duplicates? you can check duplicates by doing something like length(array) <> length(union(array, array)) in which you're comparing the length of the original against the distinct set of values in your list. 

Not an efficient way, but you could also do something like:

a!forEach(
  union(array, array),
  {
    'value': fv!item,
    'count': count(wherecontains(fv!item, array))
  }
)

What kind of validations are you trying to add - just the fact that there are duplicate values or do you need to specify directly which ones are the duplicates? you can check duplicates by doing something like length(array) <> length(union(array, array)) in which you're comparing the length of the original against the distinct set of values in your list. 

Not an efficient way, but you could also do something like:

a!forEach(
  union(array, array),
  {
    'value': fv!item,
    'count': count(wherecontains(fv!item, array))
  }
)