The other posters explained how Netzavim-Vayeilech works.
Vayakhel-Pekudei, Tazria-Metzora, Acharei Mos-Kedoshim and Behar-Bechukosai are generally used to fill the four extra weeks in a leap year (in non-leap years they’re together, but in a leap year, where we need for more Parshiyos, we separate them).
Chukas-Balak is a makeup double Parsha if the 8th day of Pesach or the 2nd day of Shavuos are Shabbos – since Eretz Yisroel is one week ahead of America (because they didn’t have that extra day of Yom Tov, so they lained the regular Parsha), we double up Chukas and Balak to catch up with them.
Finally, Mattos-Maasei is almost always a double Parsha, except for when we need to separate them to make sure that Tisha B’av falls between Devarim and Va’eschanan (like this year).
I hope I got them all right. Corrections are welcome and appreciated.