This type of 'blogging' on the site is officially sanctioned and encouraged as well as supported by the user interface:
Checking the box brings up an editor to compose an answer, allowing you to submit both the question and your answer at once.
As long as the question and answer meet our quality criteria, we welcome the contribution. Instances where duplication comes into question should be evaluated on a case by case basis. If the self answered 'blog style' question and answer provide substantially more context and information than something that exists, we'll obviously favor the better content when deciding if something else needs to be closed in light of the new contribution.
And yes, the person taking the time to share their knowledge should receive reputation for doing so, provided that the knowledge is reasonably composed, relevant and useful - just like any other question or answer.
Don't treat these types of questions any differently when deciding if something warrants a flag. Flags that just point out that someone simply used the facilities that we have in place will probably be declined.