The reason I ask this is I am in the process of doing a competitive analysis of four websites and two of the sites duplicate the same menu items on both their top navigation (drop downs) and left-hand navigation (the items in the drop down).
There are probably two main schools of thought behind this, but again it depends on how applicable it is to your website or service.
One school of thought is about consistency when providing navigation for users. You may find that some top and side navigation are duplicated exactly, some have more or less content, but they maintain the same structure/hierarchy to allow the users to always know where to go to find things.
Another school of thought is that the top and side navigation should be used for different types of navigational activities, so that one ends up being designated as a top level navigation, and the other becomes a secondary navigation or is context sensitive to the content on the page.
Things that will complicate your decision making process include:
- How much content your users will have to navigate through, as there are different limits to how you can fit content into a top horizontal versus a side vertical navigation
- Whether you have a footer that also has a navigation component, as this will also add to the complexity of the overall navigation strategy for the website and for the users
- Whether your site is designed specifically for phones, tablets or desktop as some of the navigation strategies don't work well (e.g. in mobile phones doing both a top and side navigation leaves very little room for content)
- Whether you want to differentiate the user experience from your competitors, because what they are doing might not be the best design, depending on their systems and specific business/user requirements
I'd suggest not to have duplicate links as it requires more cognitive process as it provides more choices and creates confusion to the users.
"Providing redundancy on webpages can sometimes help people find their way. However, redundancy increases the interaction cost. Duplicating links is one of the four major dangerous navigation techniques that cause cognitive strain. Even if you increase traffic to a specific page by adding redundant links to it, you may lose return traffic to the site from users who are confused and can’t find what they want." (from NN/g - see link below.)
Here is the supporting article by NN/g. You can read full article @ https://www.nngroup.com/articles/duplicate-links/
Your top menu is generally the main static links on your site, visitors might simply be unaware of a change published underneath one of the many pages that your site hosts, adding in a side menu or a feature menu is a simple way to draw attention to some important information to your visitors.
It's not wrong and it's not a bad user experience, assuming it is done right.
On a major website redesign project I worked on, usability testing revealed that users often were not even AWARE that the top-level menu item was clickable. I would give them a task to find a specific page e.g. MENU #1, they would then hover over MENU #1, not see it repeated in the dropdown, and not even realize all they had to do was click that top-level item. This was a pattern; happened with multiple testers.
So, my solution was to repeat that top-level item again as a child item. Not elegant, perhaps, but it fixed the problem for that particular menu design. It's hard to prescribe hard and fast "best practices" here -- just prototype what you think is reasonable, test on that, and iterate. What comes out of the usability testing will often surprise you.