Update UDO's using DTW cause additional row added

Dear Experts,

Im having below issue, realted to UDO.

I have created one UDO, and for the first time, i uploaded all the data using DTW and successful.

Now, im trying to update some row details inside this UDO, however, instead of update the row, the system add the details as a new row.

Below is the UDO Details :

Parent Table : BP Code, Field 1

Child Table : Code, Field 2, Field 3, Field 4, Field 5

Then, when I create DTW template, i get below template :

Parent Template : CODE, Name , Field 1

Child Template : Parent Key, LineNum, CODE, Field 2, Field 3, Field 4, Field 5

I believe, CODE, and Parent key should be the same value, which is BP Code, and lineNum can be either start with 0, or 1.

I had no idea why the end result is a new row instead of just only update the row.

Thank you in advance.