The best way to deal with a duplicate question is to redirect it.
Just click on 'edit' or 'answer' and replace the page content with this:
#REDIRECT [[New title]]
where new title is a link to the question you want to redirect to.
You should remove any categories when adding a redirect.
The advantage of making a redirect is that the same question can not be created again, and people searching for something (even if mispelled) will be able find the correct question if they follow the redirect.
When you rephrase a question, a redirect is automatically created from the original question.
Deletion is not used to remove duplicates, but only to remove offensive or inappropriate questions.
When you add a redirect, all categories are automatically removed.