Unless the CL issue is a problem for you, I would not do anything until your ticket has been flown. You have a minimum of 60 days from the date the duplicate charge actually posts (if it does) to dispute. I suggest this not because you have any obligation to wait, but because a chargeback on a ticket can easily result in DL cancelling it and then you having to spend hours on the phone to get it reinstated.

If for some reason that $600 pending item does post, simply dispute it as a duplicate charge. You can do this online with most CC issuers and there is generally an option for that or something similar. You will then see a temporary credit relatively immediately, that will restore your CL to wherever it ought to be and the temporary credit will eventually become permanent when DL does not provide documentation for a second ticket.