Simple, don't post trivial answers. By trivial, I mean an answer that just points to another question in the site with little/no supporting text.
If you want to point out that it is a duplicate, flag it as an exact duplicate of another question. Leave a comment. Or vote to close as an exact duplicate. Answers are for answers to the question, not simple suggestions to look at a different question.
If you can write a decent summary of what the answer is at the duplicate question, then add that summary. Chances are, it will probably not be converted to a comment but will receive downvotes and a possible deletion as it is not being a complete answer. Do so at your own risk.
Otherwise don't complain when it gets converted.