"Death must have occurred suddenly" translate
'La muerte debió de ocurrir repentinamente'
or 'la muerte debe haber occurrido de repente'
Which is correct and what is the difference?
3 Answers
Your second sentence is correct. De repente and repentinamente are synonymous.
You needn't put deber into the preterite, because you are using "haber" which already puts the sentence into a "perfect" or action completed tense. "must have happened" or occurred
Does this help?

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Sudden death:
La muerte súbita es la aparición repentina e inesperada de una parada cardiaca en una persona que aparentemente está sana y en buen estado.