How to avoid inserting duplicate records from source table while merge?

Just don't use Temp directily as Source but filter it for distinct values first

MERGE INTO Frames as Target 
using (SELECT DISTINCT * FROM Temp) as Source
on Source.Code=Target.Code
WHEN MATCHED THEN  UPDATE  set Target.Description=Source.Description 
WHEN NOT MATCHED THEN insert (Code,Description) values (Code,Description);

create a unique index on the target table to prevent the duplicates.

Another way to avoid inserting duplicate records from the source table while merging is to just use an UPDATE and INSERT statement if you're ever in a database that doesn't support the MERGE statement:

CREATE PROC [DW].[Load_Quest_Staff_Curricula_D] AS

---------- When Not Matched By Source ----------

INSERT INTO DW.Quest_Staff_Curricula_D 
(
    User_Id,
    Full_Name,
    Job_Type_Name,
    First_Name,
    Middle_Name,
    Last_Name,
    Office_Name,
    Office_Num,
    Start_Date,
    Date_Terminated,
    Curriculum_Name,
    Curriculum_Status,
    IsCurrent,
    DateCreated,
    DateModified
)

SELECT
s.Person_AD_Name AS User_Id,
s.Person_Full_Name AS Full_Name,
s.Person_Job_Type_Name AS Job_Type_Name,
s.Person_First_Name AS First_Name,
s.Person_Middle_Name AS Middle_Name, 
s.Person_Last_Name AS Last_Name,
s.Person_Office_Name AS Office_Name,
s.Person_Office_Num AS Office_Num,
s.Person_Start_Date AS Start_Date,
s.Person_Date_Terminated AS Date_Terminated,
s.Curriculum_Name AS Curriculum_Name,
s.Student_Curriculum_Status AS Curriculum_Status,
1 AS IsCurrent,
GETDATE() AS DateCreated,
GETDATE() AS DateModified
FROM (
    SELECT *, ROW_NUMBER() OVER (PARTITION BY Person_AD_Name ORDER BY Person_AD_Name) AS Row
    FROM SABA.SABA_Curricula 
) as s 
LEFT JOIN DW.Quest_Staff_Curricula_D T
ON T.User_Id = s.Person_AD_Name
WHERE T.User_Id is NULL
AND row = 1

---------- When Matched -----------

UPDATE T
SET T.User_Id = s.Person_AD_Name,
T.Full_Name = s.Person_Full_Name,
T.Job_Type_Name = s.Person_Job_Type_Name,
T.First_Name = s.Person_First_Name,
T.Middle_Name = s.Person_Middle_Name,
T.Last_Name = s.Person_Last_Name,
T.Office_Name = s.Person_Office_Name,
T.Office_Num = s.Person_Office_Num,
T.Start_Date = s.Person_Start_Date,
T.Date_Terminated = s.Person_Date_Terminated,
T.Curriculum_Name = s.Curriculum_Name,
T.Curriculum_Status = s.Student_Curriculum_Status,
T.DateModified = GETDATE()
FROM (
    SELECT *, ROW_NUMBER() OVER (PARTITION BY Person_AD_Name ORDER BY Person_AD_Name) AS Row
    FROM SABA.SABA_Curricula 
) as s 
LEFT JOIN DW.Quest_Staff_Curricula_D T
ON T.User_Id = s.Person_AD_Name
WHERE T.User_ID IS NOT NULL
AND row = 1

---------- When Matched -----------

UPDATE DW.Quest_Staff_Curricula_D
SET IsCurrent = 0
FROM DW.Quest_Staff_Curricula_D T
LEFT JOIN (
    SELECT *, ROW_NUMBER() OVER (PARTITION BY Person_AD_Name ORDER BY Person_AD_Name) AS Row
    FROM SABA.SABA_Curricula 
) AS s 
ON T.User_Id = s.Person_AD_Name
WHERE row = 1
AND T.User_ID IS NULL 
AND isCurrent = 1

Hope this helps.