I am using AES with GCM and a colleague told me "Repeating a VCC with the same key and a different message reduces the security of the key." He did not have any additional detail to offer.
What he meant was since the encrypted VCC is in a fixed location in a packet, bad guys could intercept the messages and correlate the two messages, somehow making it easier to crack the message or key because the VCC was repeated. (I don't understand how bad guys would know if the VCC was repeated or not...)
My understanding is the VCC is there to prevent replay attacks, nothing more, nothing less. I understand they should not be repeated.
So my question is this: does repeating the VCC with a different message and the same key actually make it easier to crack the encryption key? If so, how?