The stories are similar and the characters are similar too. In OUT we don't have time travel, but in both they talk about gangs.

With the same actor. In both dramas, the focus of the plot takes place in one room.

In both the film and the Korean drama, the two male protagonists manage to go back in time, the first by blowing out birthday candles, the second by burning incense.

In both the film and the Korean drama, the two male protagonists relive the same day in hopes of being able to save their sweetheart.

In both films, the protagonist can see a ghost-boy and helps him solve a case
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both dramas feature two men with a mentally impaired.
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the protagonists are falsely accused and end up in prison. Here cellmates become friends.
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Both very similar, they talk about boys who train to swim to achieve their dream.
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In both dramas there is a camera that can control time: in Blue Birthday it turns back time and in 9 seconds: eternal time it stops the time.

In both dramas the protagonists are afflicted because years ago they failed to save what was their first love.

In both dramas there are time travel to be able to save and win the first love.
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In both dramas you travel to the past through photographs in order to conquer your first love........