We have an supplementary sales item setup:
1. Supplementary items needs to be added for all customer
a. code = All
2. For a specific item
a. Item code = Table
b. Item = PW002
3. It is not optional
a. Optional = No
4. It is free of charge
a. Free of Charge = yes
5. It should be applied to all lines containing the item
a. Once only = No

 
CALCULATE SUPPLEMENTARY ITEMS FROM SO
When SO is created with one or more lines containing the items for which supplementary items are applicable and calculating supplementary items:
1. SO with 2 lines containing the item for which supplementary items are applicable.

2. Result after calculating supplementary items:
a. 2 lines are inserted containing the expected qty of the supplementary items
 
3. Assume customer calls and wants to change the qty, or sales takes notices an order entry error and changes the qty of the original sales lines:
a. Line 1 qty is changed from 5 to 4
b. Line 3 qty is changed from 2 to 1

4. Again calculate supplementary items and this is the result:
a. again inserted 2 lines (red square) with correct qty
b. But also the original lines with old quantity (arrow)


5. Now suppose the sales taker was distracted and forgot he already calculated the supplementary items and recalculates again:
a. D365 CSM would keep adding the lines, instead of checking the existence and updating already created lines:

 
6. There seems to be a related between the calculated supplementary items, because when canceling the original manually created SO line all related supplementary SO lines are also cancelled:


For this to happen you need to enable below setting on the customer masterdata:
Sales order defaults: Automatic cancel = Yes