The 140th mitzvah is that we are commanded to count the years, beginning with the time we ruled over Eretz Yisrael and took possession of it.1
This mitzvah of counting the shemittah years is to be fulfilled by the beth din, i.e. the Great Sanhedrin. They have the responsibility of counting each of the 50 years, just as each of us is required to count the days of the omer.
The source of this commandment is G‑d's statement,2 "You shall count seven Sabbatical years."
The Sifra says: "One might think that they count seven consecutive Sabbatical years and then make a jubilee year. Therefore the verse continues, 'seven years, seven times.' If not for these two phrases, we would not know the law."
This means that without both phrases we would not understand how to perform this mitzvah, i.e. to count just the years and to count the appropriate shemittah cycle together with it.
