If you are going to ask a question specific to Debian, you have to give it a debian tag. Fair enough. But I see many also mention the word Debian in the title as well:
How to remove dpkg in Debian?
How to remove dpkg?
Since the Stack Exchange search engine is a little limited, it really helps to have good information in the tags. But tags should be labels describing the content, not sources of additional information. Additionally, the titles are what you see if you're skimming the list of questions. It's important for them to be succinct statements of the key part of the question.
So, if a particular distribution is important to the question, yes, please put it in both places.
To be clear, I think it is fine to duplicate the tags in the title, but only when they can be worked into the titles organically and conversationally.