Whilst not a common practice, addition of a double confirmation on instruction of the issuing bank, does occur and was addressed in ICC Opinion R792 (TA.822rev).
An advising bank confirmed a credit which was subsequently also confirmed by the second advising bank.
The query questioned the possibility of a credit having a ‘double' confirmation.
As outlined in the analysis of the Opinion, this is entirely feasible. UCP 600 does not restrict credits to having only one confirming bank.
Where there is more than one confirming bank, and to avoid documents being routed through each bank and thereby delaying receipt by the issuing bank, it is common for the second confirming bank to act in the traditional confirming bank role towards the beneficiary and for the first confirming bank to act as the reimbursing bank or guarantor for reimbursement to the second confirming bank.