add an assertion when inserting an already existing element
add an assertion when inserting an already existing element

madlib/eigen_backup,mjbshaw/Eigen,rotorliu/eigen,madlib/eigen_backup,madlib/eigen_backup,rotorliu/eigen,mjbshaw/Eigen,rotorliu/eigen,madlib/eigen_backup,rotorliu/eigen,mjbshaw/Eigen,mjbshaw/Eigen