An effect “doubles” if it instructs a player to double something, or if it replaces an event with “double that much,” “twice as many,” or similar.

Double strike isn’t an effect, so it’s not doubled.

An effect that increases something by a specific amount (such as the first ability of Jaya, Venerated Firemage) isn’t doubling, even if the result is double the normal result.

If two Maro’s Gone Nuts are on the battlefield, effects that double instead octuple. If three are on the battlefield, effects that double instead sexdecuple, and so on.