how to get rows with more than 1 appearance of a specific column in relational algebra


I have a table like this:

PostId   |    Body    |    AuthorId    2             b               F   2             b               E   4             d               A   4             d               E   8             h               F  

So what I want is to get all the rows that have more that 1 appearance of PostId. Here the result would be 2 and 4 because they appear more than 1 time. I want this in relational algebra.
I have already a relation that works just fine but in this relation I use aggregation methods and I don’t really what to use count in my relation. I am wondering if there is a way to this with subtraction or division operators?
What I do for now is π PostId (σ c ≥ 2 ( γ Body; COUNT(PostId)->c R2)) to get the row with more than 1 appearance of PostId.
Thanks in advance for help