The Chinese Grammar Wiki writes:
Note that 难道 can only be used for rhetorical questions which could be answered with yes or no, and generally end with 吗.
And gives examples such as:
However, none of the examples on 标准教程HSK4上 page 105 include 吗, such as:
From the examples I've seen, I'm getting the impression that the 吗 at the end can be included or omitted without changing the meaning.
Question: Can we omit the 吗 at the end of a 难道 rhetorical question?