BUG #17018: Two versions of the same row of records are returned in one query

The following bug has been logged on the website:

Bug reference:      17018
Logged by:          Keqiang Li
Email address:      52194501011@stu.ecnu.edu.cn
PostgreSQL version: 9.6.21
Operating system:   CentosLinux release 7.4.1708 ( Core)
Description:

Schema and Initial data: 
Create Table t(a int primary key, b int);
Insert into t values(1,2);
Insert into t values(2,3);

Operation:
There are two sessions executing at the same time.

[Time0, SessonA]
> Begin;
> set transaction isolation level repeatable read;
> Select * from t where a=1;

[Time1, SessonB]
> Begin;
> set transaction isolation level read committed;
> Delete from t where a=2;
> Commit;

[Time2, SessonA]
> Insert into t values(2,4);
> Select * from t where a=2;

Here, we expect PostgreSQL Server to return a row:
2 4

However, it returns two rows:
2 4
2 3