Oracle 11g Release 2

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

CREATE TABLE JOURNEY
AS
SELECT 51 JOURNEY,
       'AAA' CHKPT1,
       'BBB' CHKPT2,
       1 LEG
       FROM DUAL
UNION ALL
SELECT 51 JOURNEY,
       'BBB' CHKPT1,
       'CCC' CHKPT2,
       2 LEG
       FROM DUAL
UNION ALL
SELECT 51 JOURNEY,
       'DDD' CHKPT1,
       'CCC' CHKPT2,
       3 LEG
       FROM DUAL
UNION ALL
SELECT 51 JOURNEY,
       'EEE' CHKPT1,
       'DDD' CHKPT2,
       4 LEG
       FROM DUAL
UNION ALL
SELECT 51 JOURNEY,
       'EEE' CHKPT1,
       'FFF' CHKPT2,
       5 LEG
       FROM DUAL
UNION ALL
SELECT 52 JOURNEY,
       'NNN' CHKPT1,
       'MMM' CHKPT2,
       1 LEG
       FROM DUAL
UNION ALL
SELECT 52 JOURNEY,
       'OOO' CHKPT1,
       'NNN' CHKPT2,
       2 LEG
       FROM DUAL
UNION ALL
SELECT 52 JOURNEY,
       'PPP' CHKPT1,
       'OOO' CHKPT2,
       3 LEG
       FROM DUAL
UNION ALL
SELECT 52 JOURNEY,
       'PPP' CHKPT1,
       'QQQ' CHKPT2,
       4 LEG
       FROM DUAL
UNION ALL
SELECT 52 JOURNEY,
       'RRR' CHKPT1,
       'QQQ' CHKPT2,
       5 LEG
       FROM DUAL;

10 rows affected

SELECT JOURNEY,
       CASE
         WHEN CHKPT1 IN (LEAD(CHKPT1, 1) OVER (PARTITION BY JOURNEY
                                               ORDER BY LEG),
                         LEAD(CHKPT2, 1) OVER (PARTITION BY JOURNEY
                                               ORDER BY LEG)) THEN
           CHKPT2
         WHEN CHKPT2 IN (LEAD(CHKPT1, 1) OVER (PARTITION BY JOURNEY
                                               ORDER BY LEG),
                         LEAD(CHKPT2, 1) OVER (PARTITION BY JOURNEY
                                               ORDER BY LEG)) THEN
           CHKPT1
         WHEN CHKPT1 IN (LAG(CHKPT1, 1) OVER (PARTITION BY JOURNEY
                                              ORDER BY LEG),
                         LAG(CHKPT2, 1) OVER (PARTITION BY JOURNEY
                                              ORDER BY LEG)) THEN
           CHKPT1
         WHEN CHKPT2 IN (LAG(CHKPT1, 1) OVER (PARTITION BY JOURNEY
                                              ORDER BY LEG),
                         LAG(CHKPT2, 1) OVER (PARTITION BY JOURNEY
                                              ORDER BY LEG)) THEN
           CHKPT2
       END CHKPT1,
       CASE
         WHEN CHKPT1 IN (LEAD(CHKPT1, 1) OVER (PARTITION BY JOURNEY
                                               ORDER BY LEG),
                         LEAD(CHKPT2, 1) OVER (PARTITION BY JOURNEY
                                               ORDER BY LEG)) THEN
           CHKPT1
         WHEN CHKPT2 IN (LEAD(CHKPT1, 1) OVER (PARTITION BY JOURNEY
                                               ORDER BY LEG),
                         LEAD(CHKPT2, 1) OVER (PARTITION BY JOURNEY
                                               ORDER BY LEG)) THEN
           CHKPT2
         WHEN CHKPT1 IN (LAG(CHKPT1, 1) OVER (PARTITION BY JOURNEY
                                              ORDER BY LEG),
                         LAG(CHKPT2, 1) OVER (PARTITION BY JOURNEY
                                              ORDER BY LEG)) THEN
           CHKPT2
         WHEN CHKPT2 IN (LAG(CHKPT1, 1) OVER (PARTITION BY JOURNEY
                                              ORDER BY LEG),
                         LAG(CHKPT2, 1) OVER (PARTITION BY JOURNEY
                                              ORDER BY LEG)) THEN
           CHKPT1
       END CHKPT2
       FROM JOURNEY;

JOURNEY	CHKPT1	CHKPT2
51	AAA	BBB
51	BBB	CCC
51	CCC	DDD
51	DDD	EEE
51	EEE	FFF
52	MMM	NNN
52	NNN	OOO
52	OOO	PPP
52	PPP	QQQ
52	QQQ	RRR

JOURNEY	CHKPT1	CHKPT2
51	AAA	BBB
51	BBB	CCC
51	CCC	DDD
51	DDD	EEE
51	EEE	FFF
52	MMM	NNN
52	NNN	OOO
52	OOO	PPP
52	PPP	QQQ
52	QQQ	RRR

JOURNEY	CHKPT1	CHKPT2
51	AAA	BBB
51	BBB	CCC
51	CCC	DDD
51	DDD	EEE
51	EEE	FFF
52	MMM	NNN
52	NNN	OOO
52	OOO	PPP
52	PPP	QQQ
52	QQQ	RRR

JOURNEY	CHKPT1	CHKPT2
51	AAA	BBB
51	BBB	CCC
51	CCC	DDD
51	DDD	EEE
51	EEE	FFF
52	MMM	NNN
52	NNN	OOO
52	OOO	PPP
52	PPP	QQQ
52	QQQ	RRR