Writing the same candidate for all choices is the same as only filling in the first choice and leaving the rest blank.
[0] https://peo.gov.au/assets/images/image-library/have-your-say...
[1] https://cdn.newsapi.com.au/image/v1/e5ace64c596cd5aa508068ac...
The ballot shown in the article explicitly says "Fill in no more than one oval for each candidate".
https://www.maine.gov/sos/cec/elec/upcoming/pdf/RCVMarkedBal...
You can vote for a single candidate across all your choices/rounds (across a single row), but voting for multiple candidates in the same choice will invalidate your ballot (down a single column). Multiples down a column would be like filling in all the circles for all candidates on a normal ballot, or even not filling any in. Same difference.
No. That would invalidate your ballot. Unless you mean the degenerate case, where you only vote for one person.