What is "Achas ba'Shanah" referring to?
Rashi, Rashbam and Targum Yonasan: It refers to Yom Kipur 1 (the only time in the year when blood was placed on the Mizbe'ach ha'Ketores
What are the implications of "Achas ba'Shanah"?
Refer to 30:10:1:1.
Shevu'os, 8b: It only atones one Kaparah and not two; It only atones once a year and not twice. 1
Yoma, 61a: In connection with "Chatas ha'Kipurim" mentioned just before it, it means that one only brings the bloode of one Par Chatas of Yom Kipur, but not of two. Consequently, if the blood spilt in the middle of the Matanos, the Kohen Gadol cannot bring another bull (ot goat) and continue with the Matanos where he left off, but must perform all the Matanos from the second bull (or goat).
See Torah Temimah, who elaborates in great detail.
Why does the Torah write "Achas ba'Shanah" twice?
Oznayim la'Torah: Because on Yom Kipur, the Kohen Gadol placed the Dam Chatas twice, once on the K'ranos, to atone for the people, and once, on the floor of the Mizbe'ach itself
Oznayim la'Torah: As implied in the word "Alav".