ABAP_DESCRIPTION The addition DISCARDING DUPLICATES> is used twice to ignore duplicate lines:
DISCARDING DUPLICATES> after itab1> prevents an exception when inserting the third line of itab1> in itab2>.
In the mapping rule, DISCARDING DUPLICATES> prevents the exception when the second line of the tabular component x2> is inserted when inserting the second line of itab1> in itab2>.