Enduring Tenacity 
Enchantment Creature — Snake Glimmer
$12.59 #787 0.85 2.1 years
Whenever you gain life, target opponent loses that much life.
When Enduring Tenacity dies, if it was a creature, return it to the battlefield under its owner's control. It's an enchantment. (It's not a creature.)
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Cards similar to Enduring Tenacity expected a card that isn't here?
Reports become test cases the matcher is graded against before an update ships. Delvefall isn't a chatbot guessing, it compares the printed rules text of every card with plain math, and examples like yours decide which differences should count.
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"Whenever you gain life, target opponent loses that much life."
Enduring Vitality86%
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"When this card dies, if it was a creature, return it to the battlefield under its owner's control. It's an enchantment."
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"Whenever you gain life, each opponent loses 1 life."
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"Whenever you gain life, each opponent loses 1 life."
Enduring Innocence86%
$3.29↓#7260.33↓2.1 years
"When this card dies, if it was a creature, return it to the battlefield under its owner's control. It's an enchantment."
Tenacious Dead85%
$0.14↓#13444↓0.20↓13.3 years↑
"When this creature dies, you may pay 
. If you do, return it to the battlefield tapped under its owner's control."
. If you do, return it to the battlefield tapped under its owner's control."
Geralf's Messenger82%
$2.37↓#8124↓0.00↓14.7 years↑
"When this creature enters, target opponent loses 2 life."
+1 more matching line
Indulging Patrician81%
$0.38↓#4301↓0.27↓6.3 years↑
"At the beginning of your end step, if you gained 3 or more life this turn, each opponent loses 3 life."
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$0.21↓#2752↓0.31↓5.5 years↑
"Whenever you gain life, each opponent loses 1 life."
South Wind Avatar80%
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"Whenever you gain life, each opponent loses 1 life."
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"At the beginning of your end step, each opponent loses X life, where X is the amount of life you gained this turn."
Professor Dellian Fel78%
$4.68↓#6701↓0.38↓0.5 years↓
"−6: You get an emblem with "Whenever you gain life, target opponent loses that much life.""
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"When this creature dies, return it to the battlefield transformed under your control."