Transact SQL :: MERGE Statement Did Not Catch Existing Row In Target Table

I am new to use MERGE statement. The MERGE cannot find any match Cardnumber in the target table.  It inserts row into an existing row on the target table causing SQL rejected with duplicate key not allowed. The CardNumber is defined as a primary key on the target table with no duplicate allowed. Below snippet stop when MERGE insert a row exists on the target. The source table contains multiple rows with the same Cardnumber because it is a transactional table with multiple redemptions. 

If MERGE cannot handle many (source) to one (target) relationship, what other method that I can change to in order to update the target GiftCard table which keeps track of gift card balance? 

Below is the error message:

Msg 2627, Level 14, State 1, Line 5
Violation of PRIMARY KEY constraint 'PK_GiftCard'. Cannot insert duplicate key in object 'dbo.GiftCard'. The duplicate key value is (63027768).

The statement has been terminated.

DDL of the target table:

CREATE
 TABLE [dbo].[GiftCard](
[CardNumber] [varchar](50) NOT NULL,
[Year] [int] NULL,
[Month] [int] NULL,
[Period] [int] NULL,
[TransAmount] [decimal](10, 2) NULL,

[Code] ....