how to get rows with more than 1 appearance of a specific column in relational algebra
Here is a relational algebra expression given $R_2$ described in the question that uses set division operator
$$
\Pi_{\text{PostId}, \text{AuthorId}}(R_2) \div \sigma_{\text{AuthorId} = E}(\Pi_{\text{AuthorId}}(R_2))
$$
$\sigma_{\text{AuthorId} = E}(\Pi_{\text{AuthorId}}(R_2))$ will return a set $\{E\}$. Then by the definition of set division operator, we can see that only $(2,E)$ and $(4,E)$ belong to the $R_2$. Thus, the above relational algebra expression will return $\{2,4\}$. Also, we observe that both 2 and 4 are the only PostIDs that have more than one appearance. Thus, the above relational algebra expression gives the right answer.