Trigger causing primary key violation - sql

Question

We have a trigger that we are trying to update so that when a record is updated, the trigger will update a second table. When we activate the change, we get an error
Violation of PRIMARY KEY contstraint 'KPRIMARY_SO_SalesOrderHeader". Cannot insert duplicate key in object 'dbo.SO_SalesOrderHeader". The duplicate key is XXXXXX.
I'm really confused by this because I don't see in the trigger where we are trying to insert a key.
ALTER Trigger [dbo].[SO_SALESORDERHEADER_onOrderChange]
ON [dbo].[SO_SalesOrderHeader]
FOR INSERT, UPDATE
AS
SET NOCOUNT ON;
--+
INSERT INTO dbo.OrderUpdateQueue (SourceId, OrderNumber, Action)
SELECT DISTINCT
N'SO', Inserted.SALESORDERNO, N'U'
FROM
Inserted
LEFT JOIN
dbo.OrderUpdateQueue ON (Inserted.SALESORDERNO = OrderUpdateQueue.OrderNumber)
AND (OrderUpdateQueue.SourceID = N'SO')
AND (OrderUpdateQueue.Action = N'U')
WHERE
(OrderUpdateQueue.[Key] IS NULL)
AND (Inserted.SALESORDERNO IS NOT NULL);
--+ New Trigger Stuff
UPDATE MO
SET MO.[ShipDate] = I.ShipExpireDate
FROM [BACKEND_db].[dbo].[Order] MO
INNER JOIN Inserted I ON I.SalesOrderNo = MO.OrderId
WHERE MO.ShipDate <> I.ShipExpireDate;
CREATE TABLE [dbo].[SO_SalesOrderHeader](
[SalesOrderNo] [varchar](7) NOT NULL
CONSTRAINT [KPRIMARY_SO_SalesOrderHeader] PRIMARY KEY CLUSTERED
(
[SalesOrderNo] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF,
ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
CREATE TABLE [dbo].[OrderUpdateQueue](
[Key] [int] IDENTITY(1,1) NOT NULL,
[SourceId] [nvarchar](50) NOT NULL,
[OrderNumber] [nvarchar](50) NOT NULL,
[Action] [nchar](1) NOT NULL,
CONSTRAINT [PK_OrderUpdateQueue] PRIMARY KEY CLUSTERED
(
[Key] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF,
ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
USE [BACKEND_db]
CREATE TABLE [dbo].[Order](
[Key] [int] IDENTITY(1,1) NOT NULL,
[OrderId] [nvarchar](10) NULL,
[ShipDate] [datetime] NULL
CONSTRAINT [PK_MasOrder] PRIMARY KEY CLUSTERED
(
[Key] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF,
ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON, FILLFACTOR = 90) ON [PRIMARY]
) ON [PRIMARY]
GO

Answer 1

How you do not post the schema of OrderUpdateQueue table, I believe the pk is the OrderNumber field. You can try to change your trigger to:
ALTER Trigger [dbo].[SO_SALESORDERHEADER_onOrderChange]
ON [dbo].[SO_SalesOrderHeader]
FOR INSERT, UPDATE
AS
SET NOCOUNT ON;
--+
INSERT INTO dbo.OrderUpdateQueue (SourceId, OrderNumber, Action)
SELECT DISTINCT
N'SO', Inserted.SALESORDERNO, N'U'
FROM
Inserted
WHERE not exists (SELECT 1 FROM dbo.OrderUpdateQueue where Inserted.SALESORDERNO = OrderUpdateQueue.OrderNumber)
UPDATE MO
SET MO.[ShipDate] = I.ShipExpireDate
FROM [BACKEND_db].[dbo].[Order] MO
INNER JOIN Inserted I ON I.SalesOrderNo = MO.OrderId
WHERE MO.ShipDate <> I.ShipExpireDate;