How to get vectors of common duplicates in a cell array?

Let's say I have a cell array of values, some of which are duplicates:

A = {'a', 'b', 'c', 'c', 'a', 'a'}

How can I get an output of the duplicate index values, grouped together? Should look something like this:

duplicates = {[1,5,6], [3,4]}

Thanks!

Answer 1

[Stephen23]

>> [~,~,X] = unique(A);
>> [B,Y] = hist(X,1:max(X));
>> C = arrayfun(@(y)find(X==y),Y(B>1),'uni',0);
>> C{:}
ans =
  1  5  6
ans =
  3  4

Or

>> [~,~,X] = unique(A);
>> V = 1:numel(A);
>> C = accumarray(X(:),V(:),[],@(v){v});
>> C(cellfun(@isscalar,C)) = [];
>> C{:}
ans =
  1
  5
  6
ans =
  3
  4

Or a simple loop.