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Regarding the Dexter example, I'm not sure it applies. Dexter doesn't faint because he's afraid of blood. he faints because it is literally the first time he had a flashback to his mother's murder, which happened to also involve a lot of blood. The scene is similar enough to trigger it. He actually blames the fainting on the blood in order to cover that he's remembered something.
Note that there are plenty of times in the series where he sees large quantities of blood and never faints or shows distress.
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