Resolving the MERGE statement attempted to UPDATE or DELETE the same row more than once error

-- begin of insert using merge
insert into dbo.tblDimSCDType2Example
( --Table and columns in which to insert the data
  SourceID1,
  SourceID2,
  Attribute1,
  Attribute2,
  Check_Sum,
  EffectiveDate,
  EndDate
)
-- Select the rows/columns to insert that are output from this merge statement 
-- In this example, the rows to be inserted are the rows that have changed (UPDATE).
select    
SourceID1,
SourceID2,
Attribute1,
Attribute2,
Check_Sum,
EffectiveDate,
EndDate
from
(
  -- This is the beginning of the merge statement.
  -- The target must be defined, in this example it is our slowly changing
  -- dimension table
  MERGE into dbo.tblDimSCDType2Example AS target
  -- The source must be defined with the USING clause
  USING 
  (
    -- The source is made up of the attribute columns from the staging table.
    SELECT 
    SourceID1,
    SourceID2,
    Attribute1,
    Attribute2,
    Check_Sum
    from dbo.tblStaging
  ) AS source 
  ( 
    SourceID1,
    SourceID2,
    Attribute1,
    Attribute2,
    Check_Sum
  ) ON --We are matching on SourceID1 and SourceID2 in the target table and the source table.
  (
    target.SourceID1 = source.SourceID1
    and target.SourceID2 = source.SourceID2
  )
  -- If the ID's match but the CheckSums are different, then the record has changed;
  -- therefore, update the existing record in the target, end dating the record 
  -- and set the CurrentRecord flag to N
  WHEN MATCHED and target.Check_Sum <> source.Check_Sum 
               and target.CurrentRecord='Y'
  THEN 
  UPDATE SET 
    EndDate=getdate()-1, 
    CurrentRecord='N', 
    LastUpdated=getdate(), 
    UpdatedBy=suser_sname()
  -- If the ID's do not match, then the record is new;
  -- therefore, insert the new record into the target using the values from the source.
  WHEN NOT MATCHED THEN  
  INSERT 
  (
    SourceID1, 
    SourceID2,
    Attribute1,
    Attribute2,
    Check_Sum
  )
  VALUES 
  (
    source.SourceID1, 
    source.SourceID2,
    source.Attribute1,
    source.Attribute2,
    source.Check_Sum
  )
  OUTPUT $action, 
    source.SourceID1, 
    source.SourceID2,
    source.Attribute1,
    source.Attribute2,
    source.Check_Sum,
    getdate(),
    '12/31/9999'
) -- the end of the merge statement
--The changes output below are the records that have changed and will need
--to be inserted into the slowly changing dimension.
as changes 
(
  action, 
  SourceID1, 
  SourceID2,
  Attribute1,
  Attribute2,
  Check_Sum,
  EffectiveDate,
  EndDate
)
where action='UPDATE';