I sometimes combine multiple essays together and rewording every paragraph in my own words + complemented with my own knowledge and style of writing.
Just don't copy an essay WORD for WORD, and how the hell are the teachers goin to be like 'thats plagiarism.' Know that theres a difference with 'common knowledge' i.e. something that everyone should know if they 'studied the topic' or somethin out of the blue that no student will know about.
Theres so many students doin the HSC, so like everyone said, similarity is always a given. It means you are gettin the right idea.