Problem 2699. From a given cell array of strings, extract the non-unique strings.

We are given a cell array of strings C. From this cell array we require to extract the subset of non-unique strings, returning the results as a column vector.

For example, if C = {'A', 'B', 'C', 'A', 'B'}, then the strings 'A' and 'B' are repeated, so the result should be {'A'; 'B'}. The order of occurrence of the strings should be preserved, so {'B'; 'A'} would not be an acceptable solution. The order of appearance of the strings is defined using the column-major format. For example, if C = {'Jack', 'Bob'; 'Dave', 'Amanda'} then the order of the strings is 'Jack', 'Dave', 'Bob', 'Amanda'.