"Save up to 50% off"

Solution 1:

It's not mathematically or grammatically correct - but its meaning is clear enough.

"Save up to 50%!" or "Savings of up to 50%!" would be correct.

Unless they are trying some subtle legal trick of actually reducing the discount from 50% to 25% and claiming that they are telling the truth and that halving the saving is "saving 50% off".

Solution 2:

Save up to %50 off

is just pleonastic, not a double negative.

Yes, it would be simpler to say

Save up to %50.

or

Get up to %50 off.

In the first statement, you're not really able to parse it as an object of '%50 off' in one direction which you then 'save' in the other direction. They just reinforce each other, but the extra 'off' is not really needed.