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Describe the Bug:
If Mirror Entity is activated more than once during the same turn, creatures that were affected by the first activation of Entity's activated ability become 0/0 upon resolution of a consecutive activation of said ability.

Example: I control a Mirror Entity and a Storm Crow, activate Mirror Entity for . Both creatures become 1/1 and a legacy effect card is attached to each of them.
I then cast cast a Whippoorwill and, after it entered the battlefield, activate Mirror Entity for . Mirror Entity and Whippoorwill become 3/3, Storm Crow becomes 0/0.

Which card did behave improperly ?
Mirror Entity

Which update are you using?(date,name)Which type(Duel,Gauntlet,Sealed Deck)

What exactly should be the correct behavior/interaction ?
Creatures controlled by Mirror Entity's controller become X/X when Entity's activated ability resolves, regardless of the number of times that ability had been activated prior to the last activation.

Are any other cards possibly affected by this bug ?
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I got something even more complicated. I had 5 other creatures in play (and a Gaea's Anthem) when I cast Mirror Entity. I then activated it once for 1, then once for 3, and then once for 4. After the 2nd activation 3 creatures died and the other 2 got pumped bigger. And after the 3rd activation a Simic Basalisk was 12/12, an Abattoir Ghoul was 3/5 and the Mirror Entity was 7/7 (the 3 creatures that died were Abominations).

I've standardized Mirror Entity ability with the current implementation of cards that alter the base P/T of creatures in 1c18c6a