There are two possibilities with this:
1. the other purchase actually did not succeed
2. you purchased one Bond for the wrong game version
If it is point 1. your money will be returned into your account. Work through this support page:
Unsuccessful payment
If it is point 2. and you definitely have confirmations from Jagex for both purchases, check both game versions. If you find the other Bond in the wrong game version, use this support page to get to a message form:
Find my Bonds