The same measure, either in form or in essential substance, may not be submitted to the people by initiative petition, either affirmatively or negatively, more often than once every two years. No attempt to repeal or alter an existing measure or portion of such measure by referendum petition may be made within two years from the last attempt to do the same. Such prohibition shall apply only when the subsequent attempt to repeal or alter is designed to accomplish the same, or essentially the same purpose as the previous attempt.