Replacing multiple occurrence by a single occurrence in SAP HANA

The code you are using \1+ just removes consecutive occurrences only. You can use the below code.

SELECT REPLACE_REGEXPR('(.)(?=.*\1)' IN '22331122' WITH '' OCCURRENCE ALL) FROM DUMMY

The output will come out to be “312”. This will remove all multiple occurrences and only last one will be kept.