These two phrases mean basically the same thing. The second phrase is used to strengthen the first. (See: [[rc://en/ta/man/translate/figs-parallelism]])
Isaiah uses rhetorical questions to make the people think deeply. AT: "A farmer does not plow the ground over and over and continually work the soil without ever sowing seed." (See: [[rc://en/ta/man/translate/figs-rquestion]])