On its own the "double jeopardy" argument holds no weight at all. Clubs know it's a continuing 3 year rolling period, if you fail once and fail again the next year, You'll get punished again. Theres no argument against that.
However, it would appear they have a strong case in this instance due to the premier leagues pissing about with rule changes and procedures for dealing with breaches. They can rightfully say they haven't had an opportunity to comply with the rules since the change in times financial records are reported and since their last charge.
Everton can simply say they were planning on selling in january to ensure they complied this year. Given that they've now been charged and it wont get dealt with instantly, they can now not sell anybody this window and just blame the uncertainty with the charges.
Looks like they could get away with further punishment on a technicality, that technicality being the premier leagues incompetence. And the timing might actually end up being beneficial for them if they do.
Would be amusing, to me at least if it played out like that and they avoided punishment given the complete myth that we somehow avoided punishment on technicalities from uefa.