It seems pretty clear to me that this answer is better than either of the answers at your question. As such, we'd rather encourage people to go to to the question with that answer rather than to the question without it.
If we made the other question a duplicate of your question, then people going to your question would never see the best answer.
Perhaps it should be obvious, but I'll state it anyway: you did nothing wrong. It's not your fault that the best answer was put on the other question. Perhaps in an ideal world, the second question would have been recognized as a dupe immediately (before any answers were posted). Then the person who posted the best answer might have posted a similar response to your question. But we're imperfect and did not do that. As someone who was reviewing on the site at that time, I apologize. We failed in our review and left it to someone else to fix our mistake, incidentally causing you to get stepped on.