I'm confused. When I look at jisho.org it seems to give the kanji for this as:
but it has two hiragana versions.
Can someone explain to me what Kanji would normally be used for this and why are there two different hiragana.
Both readings are correct even though it is perfectly valid to write 「おととし」 in hiragana.
「おととし」 is less formal and is used heavily in our daily lives.
「いっさくねん」 is more formal and business-like; therefore, it is used less often than 「おととし」 is.
「おととし」 is the original Japanese word (if you could tell from its "softer" kun'yomi sounds) and 「いっさくねん」 is the "Chinese-like" reading of the word. 「いち」, 「さく」 and 「ねん」 are all on'yomi.