I can use Spark Double to have two Amass tokens
Not only that but while in play, Chandras Incinerator effectively doubles your damage spells by copying any non-combat damage done to an opponent and allowing you to then use that to strike down creatures or planeswalkers they control
This actually came to an end during the second Phyrexian Invasion when the ship itself fell to Phyrexian corruption, and its now part of the machines armies
Also, you can pay two mana and tap them to double the number of each counter on an Artifact, Creature, or Land
Brawl Legal Vintage Legal Timeless Legal Commander Legal Pauper Not Legal Oathbreaker Legal Penny Not Legal Toolbox Buy This Card Notes and Rules Information for Attuned Hunter: If multiple cards leave your graveyard at the same time, Attuned Hunters last ability will trigger only once
Likewise, the mana-doubling effect on A Display of My Dark Power can lead to untap loops, usually with lands that are also artifacts or creatures