Edit - nevermind this post, I've made it to remind my future self I've found this someday...
Both their songs have the same few sounds at the beginning
Avril - I'm with you
Camila - Crying in the club
Listen to the only first couple of seconds, I'm curious if you could hear it or it's just me :)
Although it may sound the same at first,they are different notes played on the same instrument,Bass(contrabass) I think...
I think you're right! I don't have much musical ear as they say in our country so I don't get the differences so well :D