Firstly, I agree that using your own ideas in more than one response is fine, so long as they are relevant to the question to which you are responding.
Secondly, even if a single marker does see both responses, what are the chances that they remember that it is from the same student? Yes, the handwriting is the same but they will read a lot of essays and remembering that a similar argument was made in the same handwriting is unlikely, and they only have your student number, not a name or school or anything like that.
Also, there are efforts made to make sure that marking is consistent. If I recall correctly, each essay is double marked and when there is significant disagreement, it goes to a pilot marker for a third marking. No marks are recorded on the papers themselves, so the marker won't know if they are the first or second marker, nor what the other marker (if there has been one) gave.
In subjects like Maths and Sciences, there are papers included that have been marked by pilot markers that come past each marker to check that everyone is awarding the same marks for each question - and markers won't know which papers these are. Inconsistencies in marking are then investigated and addressed, if necessary by having all the papers from a marker who is grading differently re-assessed.
The HSC system goes to considerable effort to minimise differences between markers as a source of differences between student results.
