SQL & PL/SQL

I got a "ORA-00001: unique constraint (user.indexname) violated" error and it surprises me, as I don't see how that error can happen on an index (that is not unique).

The message:

ORA-00001: unique constraint (USER.TAB_LIST_PROP_PROPID_IDX) violated

db version (excerpt from lsinventory):

Oracle Database 11g                                              
11.2.0.4.0
There are 1 products installed in this Oracle Home.
Interim patches (2) :
Patch  17501296 
: applied on Mon Jun 15 19:02:17 CEST 2020
Unique Patch ID:  18802658
   Created on 10 Apr 2015, 16:40:58 hrs PST8PDT
   Bugs fixed:
17501296
Patch  28204707 
: applied on Mon Jun 15 18:55:06 CEST 2020
Unique Patch ID:  22456102
Patch description:  "Database Patch Set Update : 11.2.0.4.181016 (27734982)"
   Created on 20 Sep 2018, 00:18:41 hrs PST8PDT

code excerpt:

CREATE TABLE TAB_LIST_PROP (
     ListId INTEGER  NOT NULL ,
     PropId INTEGER  NOT NULL );
ALTER TABLE TAB_LIST_PROP ADD CONSTRAINT TAB_LIST_PROP_FK_LIST  FOREIGN KEY 
    (ListId) REFERENCES TAB_LIST (ListId);
ALTER TABLE TAB_LIST_PROP ADD CONSTRAINT TAB_LIST_PROP_FK_PROP  FOREIGN KEY 
    (PropId) REFERENCES TAB_PROP (PropId);
CREATE INDEX TAB_LIST_PROP_IDX        ON TAB_LIST_PROP (ListId ASC);
CREATE INDEX TAB_LIST_PROP_PROPID_IDX ON TAB_LIST_PROP (PropId) ;  -- <----- this is the index from the error message
--------------
create package body foo is
c_true constant char(1) := '1';
c_false constant char(1) := '0';
procedure createListEntry(TopicId INTEGER, 
 p_1 number,
 p_2 number,
 p_3 number,
 PropId number,
 p_id out number
 )
is
    nextID NUMBER(19,0);
begin
    SELECT TAB_LIST_SEQ.nextVal into nextID FROM dual;
    insert into TAB_LIST (ListId, 
     TopicId, 
     more,
     columns,
     here,
     PropId)
      values (nextID,
     TopicId,
     p_1,
     p_2,
     p_3,
     PropId);
    p_id := nextID;
    INSERT INTO TAB_LIST_FOO ... -- this is very similar to the next INSERT and executes without problems, so it omitted 
    -- this statement throws the error:
    INSERT INTO TAB_LIST_PROP (ListId, PropId) SELECT DISTINCT nextID as ListId, PropId 
    from TAB_TOPIC
    LEFT JOIN u_am ON u_am.TopicId=TAB_TOPIC.TopicId
        and u_am.Flag1 /* CHAR(1) */ =c_false and u_am.Flag2 /* CHAR(1) */ =c_true
    LEFT JOIN  u_oam ON u_oam.oamId=u_am.oamId
        and u_oam.Flag1=c_false and u_oam.Flag2=c_true
    LEFT JOIN ARPM on u_oam.AppRolPrmId = ARPM.AppRolPrmId
       and ARPM.Flag1 = c_false and ARPM.Flag2 = c_true
    WHERE TAB_TOPIC.TopicId=createListEntry.TopicId and PropId /* a column in the table ARPM */ is not null;
end;