Delete duplicate keys with relation algebra

Hi I am new to databases and relational algebra. I was wondering if there is a way to remove the tuples from a table using relational algebra that have the same keys but different value. e.g. I want to keep only [1, 5] and [4, 9] but remove everything else. Key | Value -------|------- 1 | 5 2 | 6 2 | 7 2 | 8 4 | 9 Thanks.