Double-dipping is when a spell or ability gets to gain the same benefit twice. The reason this is relevant to Balance Druid is because of our Mastery effect and the Astral damage type. Our Mastery increases the damage of our Eclipse, but it only affects the active damage Eclipse type. So, in Lunar Eclipse you gain an Arcane damage buff and in Solar you gain a Nature damage buff. When you use Celestial Alignment, you gain the benefits of both Eclipses simultaneously. Astral damage is the combination of both Arcane and Nature damage, meaning when we have both Eclipses up we gain the Mastery effect twice, thus 'Double-dipping'. Starsurge, Starfall, Stellar Flare, Shooting Stars, Fury of Elune, and New Moon are all considered Astral damage.