SQL error.. Please Help

I get this error: Msg 8156, Level 16, State 1, Line 11
The column 'taskname' was specified multiple times for 'a'.
Msg 8156, Level 16, State 1, Line 20
The column 'taskname' was specified multiple times for 'b'.

This is the query but not sure how to fix it.;

SELECT distinct a.taskname, a.PackageName, a.TaskId, a.LogTime, datediff(second,b.LogTime, a.LogTime) DURATION_in_sec
FROM
(select PackageName, TaskName, TaskId, LogTime,
substring(taskname,0,patindex('%Begin%',taskName)) taskname
from SSIS_ExecutionLog where PackageGroup = 'CapacityPlanning' and taskname like '%Begin%'
AND LogStatus <> 'Skip'
union all
select PackageName, TaskName, TaskId, LogTime,
substring(taskname,0,patindex('%End%',taskName)) taskname
from SSIS_ExecutionLog where PackageGroup = 'CapacityPlanning' and taskname like '%End%') a
JOIN
(select PackageName, TaskName, TaskId, LogTime,
substring(taskname,0,patindex('%Begin%',taskName)) taskname
from SSIS_ExecutionLog where PackageGroup = 'CapacityPlanning' and taskname like '%Begin%'
AND LogStatus <> 'Skip'
union all
select PackageName, TaskName, TaskId, LogTime,
substring(taskname,0,patindex('%End%',taskName)) taskname
from SSIS_ExecutionLog where PackageGroup = 'CapacityPlanning' and taskname like '%End%') b
ON
a.TaskID = B.TaskID
AND datediff(second,b.LogTime, a.LogTime) > 0

Any advice is appreciated!