Said commission shall, on the first Tuesdays in April and October of each year, or oftener if it shall be deemed necessary, under such rules and regulations as may be prescribed by the council, hold examinations for the purpose of determining the qualifications of applicants for positions, which examination shall be practical and shall fairly test the fitness of persons examined to discharge the duties of the position to which they seek to be appointed. Said commission shall as soon as possible after such examination certify to the council double the number of persons necessary to fill vacancies, who according to its records have the highest standing for the positions they seek to fill, and all vacancies which occur that come under the civil service prior to the date of the next regular examination shall be filled from said list so certified; provided, however, that should the list become exhausted, then the council or the head of the proper department may temporarily fill a vacancy, but not to exceed thirty days.