Inquisitor's Flail

• WOTC Official Reminder Textofficial rules
  Equip — Equip [cost] ([cost]: Attach to target creature you control. Equip only as a sorcery.)

• WWizards of the Coast2011-09-22
  Official Ruling
  If a creature is equipped with a second Inquisitor’s Flail, combat damage dealt by and dealt to that creature will be multiplied by four. A third Inquisitor’s Flail would multiply the combat damage by eight, and so on.
• WWizards of the Coast2011-09-22
  Official Ruling
  If you divide the combat damage dealt by the equipped creature, perhaps because the creature has trample or is dealing combat damage to multiple creatures, you’ll divide the original amount and then double the results. For example, if a 5/5 creature with trample is blocked by a 2/2 creature, you can assign 2 damage to the blocker and 3 damage to the defending player. These amounts are then doubled to 4 and 6 damage, respectively. You can’t double the damage to 10 first and then assign 2 to the creature and 8 to the player.

Rulings via Scryfall