How to know if logreader agent already exists?
Below T-SQL will give you the log reader agent. You can use the agent name or job_id to drop/delete the log reader agent.

    SELECT SERVER
    ,[command]
    ,sj.job_id
    ,[NAME]
FROM msdb.dbo.sysjobs sj
INNER JOIN msdb.dbo.sysjobsteps sjs ON sjs.job_id = sj.job_id
    AND subsystem = 'logreader'