You can't anymore when they did the major update to the site you could select up to three out of about 50 categories, but users were overwhelmed by that so the merged categories and created other so that users were less overwhelmed. After that change you could only select one.
There is only one topic per question. If you post the same question more than once, the duplicates will be removed per posting guidelines which can be found in our FAQ - http://www.girlsaskguys.com/faq/posts
It's against site rules to ask the same question more than once, which you would need to do for it to appear in more than one topic. Before the most recent update you where able to select up to 3 topics for your question, and it would show up in those topics, now they reverted back to just one topic per question.