	
	_D_e_t_e_r_m_i_n_e _D_u_p_l_i_c_a_t_e _E_l_e_m_e_n_t_s
	
	     duplicated(x)
	
	_V_a_l_u_e:
	
	     This function determines which elements of a vector are
	     duplicates of elements with smaller subscripts, and
	     returns a logical vector indicating which elements are
	     duplicates.
	
	_E_x_a_m_p_l_e_s:
	
	     # extract unique elements
	     x[!duplicated(x)]
	
