From my part (since it was I who did this), you had answered the question already on the duplicate.
Since you effectively had recognised that the question was a duplicate and had correctly answered on the correct question, my role was simple - to play the janitor and thus cleanup by removing the answer.
There was no reason to merge here since you had already answered the question.
Most traffic to StackExchange sites is via search engines. To redirect users to the correct question, StackExchange uses the policy of redirection as explained here:
Now ordinarily I would normally leave questions and answers alone unless they start to receive high traffic (visits). In this case, you had answered the question on the duplicate - thus leaving two answers on separate questions would have been nonsensical - you potentially would be benefitting twice with the same answer.
Hence the janitorial rule here applies - cleanup and move on to find similar stuff to tidy up.