At first glance, this might just seem like the unnecessary introduction of an ability word
If an effect instructs you to "double" a creature's power, that creature gets +X/+0, where X is its power as that effect begins to apply
Most MTG sets have their own, corresponding Starter Kit for example, the upcoming Lord of the Rings MTG set will have its own two player Starter Kit, complete with two MTG Arena code cards
I'd run Garruk's Harbinger in just about any Creature-heavy deck
Loot gets double strike, vigilance, and haste, making it a massive threat as soon as it comes into play
Check the photos for full details on the deck list and card quality