On both Star Trek series where he appears, O'Brien's standard coffee order seems to be "black, double sweet." In the TNG episode "Rascals," Keiko even knows that it's his standard order.
However, in the DS9 episode "Whispers," he repeatedly orders his coffee "Jamaican blend, double strong, double sweet."
This happens four or five times in the episode, including right at the beginning. He never orders coffee this way in any other episode.
Additionally, the plot twist in that episode is that
the O'Brien we're seeing is actually an unknowing duplicate, created to sabotage the peace-talks-of-the-week.
One could suppose that
the duplicate O'Brien has slightly different taste in coffee than the real one, and this is intended to be a subtle clue that he's a duplicate.
Then again, it's also possible that he's just drinking stronger coffee than usual because he spends the entire episode sleep-deprived.
Is there any evidence (in canon, or in creator commentary) that would distinguish between these two theories?