code: public static boolean cs_dupl(DZcs A)
  	{
  		int i, j, p, q, nz = 0, n, m, Ap[], Ai[], w[] ;
  		DZcsa Ax = new DZcsa() ;
  		if (!CS_CSC (A)) return (false) ;		/* check inputs */
  		m = A.m ; n = A.n ; Ap = A.p ; Ai = A.i ; Ax.x = A.x ;
  		w = new int [m] ;				/* get workspace */
  //		if (w == null) return (false) ;			/* out of memory */
  		for (i = 0 ; i < m ; i++) w [i] = -1 ;		/* row i not yet seen */
  		for (j = 0 ; j < n ; j++)
  		{
  			q = nz ;				/* column j will start at q */
  			for (p = Ap [j] ; p < Ap [j+1] ; p++)
  			{
  				i = Ai [p] ;			/* A(i,j) is nonzero */
  				if (w [i] >= q)
  				{
  					Ax.set(w [i], cs_cplus(Ax.get(w [i]), Ax.get(p))); /* A(i,j) is a duplicate */
  				}
  				else
  				{
  					w [i] = nz ;		/* record where row i occurs */
  					Ai [nz] = i ;		/* keep A(i,j) */
  					Ax.set(nz++, Ax.get(p)) ;
  				}
  			}
  			Ap [j] = q ;				/* record start of column j */
  		}
  		Ap [n] = nz ;					/* finalize A */
  		w = null ;
  		return (cs_sprealloc (A, 0)) ;			/* remove extra space from A */
  	}
docs: Removes and sums duplicate entries in a sparse matrix.
  
  @param A
  column-compressed matrix
  @return true if successful, false on error