Generally, bloggers do write a single post in 2 or 3 categories. But doesn’t it count as duplicacy? Is this not a sin in Google’s eyes. This is the question that comes in the mind of every new blogger. Me too also thought same when I started blogging initially. Some do think that almost all do follow this strategy, so it might be acceptable by Search engines. But why, after all it’s a duplicate. The reason being that those blogs are using a good SEO plug-in that overcomes this issue. If you are not using any such plugin, then beware you are asking Google to punish you.

Example: Say you have a domain www.abc.com and you are publishing a post in 3 categories – category1, category2, category3.
So, now the urls of that single post become
www.abc.com/category1/post.html
www.abc.com/category2/post.html
www.abc.com/category3/post.html

Now go to the View–>Source of browser for all these 3 links.
You will notice these all would contain a unique code as similar to below.

<link rel=”canonical” href=”http:// www.abc.com/category1/post.html ” />

It signifies that even the post can be accessed by 3 ways, the original link is of category1, the other 2 are just its replica. So, Google, Yahoo, Bing etc. all search engines will consider them as replica itself and will not punish you.
Any good seo plugin will take care of this issue.

But if you don’t find this canonical link in your source code, it means your plugin doesn’t provide the canonical functionality and thus either you need to install any other canonical functionality provider plugin or STOP publishing in multiple categories.