you in the past used that other code, you can't use a code twice. That second code was not emailed to you I bet, all codes emailed to you are usable one time by you. Some are generic codes that everyone can use one time, but in that list in some other forum post someone posted some codes that are not generic but specific to his account, which will never work for you. But if you were on the email list at that time then you got a code for your account that corresponds to that one, and if you were not on the email list at that time then 'no code you you'.
Technically you are supposed to be signed up for emailings of codes and nobody should be posting old codes. And even if the Devs are ok with people posting old codes the posters and the users have to realize that some codes will never work for you and that no matter what, if you used that code or if you used your specific version of tha code then you are already done on that code.
i hope this is understandable but I am sure others will reword anything that I might have wrong or confusing.

http://www.codenameentertainment.com/?page=idle_email

then one of the codes he sent you was a public code that all users could use and the other was of the kind that was specific to only his account and he used it.
Of the something like 20 codes that have been emailed to us since the initiation of the subscribing process on August 6, about 5 or 6 have been public, all others are a one time per specific account kind. You should subscribe if you want yourself to be emailed these codes. the way to subscribe is listed somewhere in these forums.