From Oracle 12, you can use MATCH_RECOGNIZE to perform row-by-row comparisons and aggregate the adjacent duplicates and then you can use LISTAGG to aggregate the unique values:

SELECT col_key,
       LISTAGG(value, ' | ') WITHIN GROUP (ORDER BY mno) AS col_value
FROM   (SELECT t.*,
               ROWNUM AS rn -- You need to provide a way of getting this order!
        FROM table1 t)
MATCH_RECOGNIZE(
  PARTITION BY col_key
  ORDER     BY rn
  MEASURES
    MATCH_NUMBER()   AS mno,
    FIRST(col_value) AS value
  PATTERN (same_value+)
  DEFINE
    same_value AS FIRST(col_value) = col_value
)
GROUP BY col_key;

Which, for the sample data:

CREATE TABLE Table1 (col_key, col_value) AS
SELECT 'key1', 'value1' FROM DUAL UNION ALL
SELECT 'key1', 'value2' FROM DUAL UNION ALL
SELECT 'key1', 'value3' FROM DUAL UNION ALL
SELECT 'key1', 'value2' FROM DUAL UNION ALL
SELECT 'key1', 'value2' FROM DUAL UNION ALL
SELECT 'key1', 'value2' FROM DUAL UNION ALL
SELECT 'key1', 'value1' FROM DUAL UNION ALL
SELECT 'key1', 'value1' FROM DUAL UNION ALL
SELECT 'key1', 'value3' FROM DUAL UNION ALL
SELECT 'key1', 'value3' FROM DUAL UNION ALL
SELECT 'key1', 'value3' FROM DUAL UNION ALL
SELECT 'key1', 'value2' FROM DUAL;

Outputs:

COL_KEY	COL_VALUE
key1	value1 | value2 | value3 | value2 | value1 | value3 | value2

db<>fiddle here