SQL Query Creates Duplicated Results

My task is to produce a report that shows the on time delivery of products to consumers. In essence I have achieved this. However, as you will see only some of the data is accurate.

Here is our test case: we have a sales order number '12312.' This sales order has had 5 partial shipments made (200 pieces each). The result is shown below from our DUE_DTS table.

Due Dates table data

The following code gives me the information I need (excluding due date information) to show the packing details of the 5 shipments:

DECLARE @t AS TABLE ( 
                         CUSTNAME char(35), 
                         SONO char(10), 
                         INVDATE date, 
                         PACKLISTNO char(10), 
                         PART_NO char(25), 
                         SOBALANCE numeric(9,2)
                    )

INSERT INTO @t
SELECT DISTINCT c.CUSTNAME, s.SONO, p.INVDATE, p.PACKLISTNO, i.PART_NO, q.SOBALANCE
FROM [manex].[dbo].[SODETAIL] 
INNER JOIN [manex].[dbo].[SOMAIN] s ON s.SONO = SODETAIL.SONO
INNER JOIN [manex].[dbo].[CUSTOMER] c ON c.CUSTNO = s.CUSTNO
INNER JOIN [manex].[dbo].[INVENTOR] i ON i.UNIQ_KEY = SODETAIL.UNIQ_KEY
INNER JOIN [manex].[dbo].[DUE_DTS] d ON d.SONO = s.SONO
INNER JOIN [manex].[dbo].[PLMAIN] p ON p.SONO = s.SONO
INNER JOIN [manex].[dbo].[PLDETAIL] q ON q.PACKLISTNO = p.PACKLISTNO
WHERE s.SONO LIKE '%12312'

SELECT * FROM @t

Here is a screenshot of the results from running this query:

Query Result

Now is when it should be time to join my due dates table (adding in the appropriate column(s) to my table definition and select statement) and make DATEDIFF comparisons to determine if shipments were on time or late. However, once I reference the due dates table, each of the 5 shipments is compared to all 5 dates in the due dates table, resulting in 25 rows. The only linking column DUE_DTS has is the SONO column. I've tried using DISTINCT and variations of the group by clause without success.

I've put enough together myself to figure joining the DUE_DTS table on SONO must be causing this to happen, as there are 5 instances of that value in the table (making it not unique) and a join should be based on a unique column. Is there a workaround for something like this?