http://revach.net/article.php?id=2384

Section: Parshas HaShavua   Category: Quick Vort
Parshas Korach: Your Son the Bechor, Twice The Price

אַךְ פָּדֹה תִפְדֶּה, אֵת בְּכוֹר הָאָדָם, וְאֵת בְּכוֹר-הַבְּהֵמָה הַטְּמֵאָה, תִּפְדֶּה  "But you must redeem the firstborn of man and redeem the firstborn of unclean beasts"  Why only for man is there a double expression, פָּדֹה תִפְדֶּה and not for animals? 

The Luach Erez answers, based on the Gemara in Bechoros (12b). The Gemara says, if a person sets aside a sheep to redeem his first-born donkey, and the sheep dies, he is not required to replace it. However, if the money one sets up to redeem his son is lost, one must provide replacement money (Bechoros 51a)

Therefore, regarding the redemption of man, the Torah uses a double expression to indicate that there is a possibility for a double expenditure: however, this is not so in regard to the redemption of an animal.