I think it's fine to cross-post answers if they're still useful and relevant, and copying the full text should be encouraged.
Link-only answers are bad, even if they link to another Stack Exchange site. If the answer on Stack Overflow gets deleted, or someone edits it to be more relevant to the SO question, we'll lose an answer!
Stack Exchange user ...
Flag the answers for moderator attention with a comment, or flag them as spam if they're really bad.
For these answers in particular:
I think if an answer happens to include a link to a tutorial you made, that's OK. However, if you're creating answers to direct users to a tutorial you made, that's promotion and not what this site is for.
I deleted some of ...
Upvotes reflect the number of people who find the answer useful. This is only sometimes related to the amount of effort you put in the answers (although some people do vote on answers that show effort, but anyone can vote however they want). In a sense the SE system rewards "good" answers, for a certain definition of good (which of course may not match your ...
It would be better to link to the original answer and explain in detail how it can be adapted to the question. However, since it's an answer and not a question, I would prefer to let the voters decide the quality of its composition. If another answer is more clear, but states the same facts, the overall better answer should win out democratically.
Self accepted answers do not count, authoritative answer here and here, where it also states that only answers older than 10 days (with no votes) are considered. Also wiki answers and deleted answers are not considered.
See also: List of all badges with full description.
If one solution is correct and the other solutions are not correct, this probably means that the question is not precise enough.
The check-mark to accept the answer says:
[...] because it solved your problem or was the most helpful in finding your solution.
And the upvote text says:
This answer is useful.
We see that the concept of "correct answer" ...
I usually downvote the answer and leave a comment. If there is already a comment regarding the issue, I downvote the answer and upvote the comment.
I do this because the downvote button text says:
This answer is not useful.
which is the truth, as there is already one covering the issue at hand; it's not useful to have a second one.
I do this also ...