Hi bzh, I understand that you are writing multiple sources to the same target table on DLT and you are following the SCD type 1 process as documented (https://docs.databricks.com/en/delta-live-tables/cdc.html#process-scd-type-1-updates). However, the way how you declared two dlt.apply_changes from two different sources triggered this error. Firstly, you can narrow down the issue to troubleshoot by removing the 1st or the 2nd apply_changes declaration, so that you can test it with only one declaration and see if the same error persists. If that doesn't throw you an error, then try to change your code to declare one apply_changes function with two sources. The apply_changes function allows you to specify multiple sources by providing a list of tables or views as the source parameter. Please refer to this sample code:

import dlt

@dlt.table
def source1():
    return dlt.read("table1")

@dlt.table
def source2():
    return dlt.read("table2")

@dlt.table
def target():
    return dlt.apply_changes(
        target="target_table",
        source=["source1", "source2"],
        keys=["key_column"]
    )

Hope this helps, and happy coding on DLT, bzh!