If every answer on the duplicate is also a valid answer on the original, then it makes sense to move them. Sometimes the duplicate is more specific than the original, and answers on that duplicate don't cover all the cases in the original; in that case I don't think it makes sense to move. It also night not make sense if both questions have collected approximately the same set of (duplicate) answers, which sometimes happens.
So before taking this action, somebody needs to review both questions and all answers to determine if they are good candidates for consolidation. It would be best if such review came from the community, to both share the work and tap the right experts.
Moderators can merge questions, which has the effect you're looking for. A merger leaves one question with all the answers and the other question as a merge stub with a link to the other question. Because the questions are true duplicates, the "surviving" master question can be either, so we'd prefer to take the one that is better, whatever that means in a particular context. A question can also have only one accepted answer, so if both questions have accepted answers at the time of the merge, one of them is going to lose that status. So, again, community input is helpful here.
I suggest using meta to assess individual cases. If you see a true duplicate with answers that would benefit from being all in one place, bring it up and see what the community thinks about merging them. Try to get consensus on the decisions that need to be made. Then ask a moderator to pull the trigger.
Thus far we have had only two mergers on this site (1, 2), and they were exact duplicates asked by the same user. But other sites have had mergers that were asked by different people with different wording, so this is something that is done sometimes. Mi Yodeya, for example, has 72 mergers out of 21k questions. Meta.SE has 81 mergers out of 81k questions.