Welcome to #CMAskTheJudge—and to my 100th video! This time we want to know how Doubling Season and Vorinclex, Monstrous Raider affect planeswalkers. Comment on our video (or below) with your answer and then check whether you got it right or wrong!
When you control Doubling Season and play a planeswalker, it will enter the battlefield with twice the regular number of loyalty counters. Putting loyalty counters on a planeswalker entering the battlefield is an effect, a replacement effect, to be precise. Doubling Season sees this effect and modifies it accordingly. However, putting counters on planeswalkers to activate their plus abilites is a cost, not an effect, and Doubling Season doesn't modify costs.
It gets interesting when you control both Doubling Season and Vorinclex and cast a planeswalker. The planeswalker will enter the battlefield with four times the regular number of loyalty counters, and planeswalkers will also get four times the number of counters when you activate their plus abilities. The reason is that the cost is affected by Vorinclex's replacement effect doubling the original counters, at which point Doubling Season sees an effect trying to put counters on a planeswalker and doubles that.