Finally, congratulation to myself and BIG THANKS to Steve !
However, I modified the expression:
AND( *** other validation (1) ***, *** other validation (2) ***, ISBLANK( FILTER( "table", AND( [key-column] = [_THISROW].[key-column], [_ROWNUMBER] <> [_THISROW].[_ROWNUMBER] ) ) ) )
Invalid value error
IFS( *** other validation error (1) ***, "error message (1)", *** other validation error (2) ***, "error message (2)", TRUE, "Duplication found by combination of 🔑 fields !" )
Notes: The key fields have the key symbol in their captions like this,
This does not prevent the system error message shown at the top of form. But there is a better guideline for users (who are not familiar with the relational database terms) of what they need to do.