Why are there multiple rows in V$SESSION_LONGOPS when running an RMAN backup?

Answer:
When performing an RMAN backup, V$SESSION_LONGOPS will be populated with two types of rows:

Detailed rows: Captures the progress of individual job steps.

Aggregated rows: Captures and summarizes the progress for the individual step of the backup job, as they complete. They are updated only after each detailed step has finished.

Example:
Below I am querying the V$SESSION_LONGOPS after an incremental backup level 0 has started, using 6 channels. Notice that there are both individual rows and an aggregated row present:

SELECT SID,SERIAL#,OPNAME,TARGET_DESC,TOTALWORK,SOFAR,UNITS,START_TIME,TIME_REMAINING,ELAPSED_SECONDS,MESSAGE
FROM V$SESSION_LONGOPS 
WHERE  TOTALWORK <> 0
AND SOFAR <> TOTALWORK
ORDER BY SERIAL# ASC;

SID	SERIAL#	OPNAME	TARGET_DESC	TOTALWORK	SOFAR	UNITS	START_TIME	TIME_REMAINING	ELAPSED_SECONDS	MESSAGE
203	7409	RMAN: incremental datafile backup	Set Count	4096000	1004926	Blocks	01.08.2014 12:28:28	557	181	RMAN: incremental datafile backup: Set Count 137085: 1004926 out of 4096000 Blocks done
398	15079	RMAN: incremental datafile backup	Set Count	3018368	1122302	Blocks	01.08.2014 12:28:28	306	181	RMAN: incremental datafile backup: Set Count 137088: 1122302 out of 3018368 Blocks done
439	17451	RMAN: incremental datafile backup	Set Count	2560000	1127038	Blocks	01.08.2014 12:28:29	229	180	RMAN: incremental datafile backup: Set Count 137089: 1127038 out of 2560000 Blocks done
357	31627	RMAN: incremental datafile backup	Set Count	3044608	1115518	Blocks	01.08.2014 12:28:28	313	181	RMAN: incremental datafile backup: Set Count 137087: 1115518 out of 3044608 Blocks done
476	55467	RMAN: incremental datafile backup	Set Count	2557440	940542	Blocks	01.08.2014 12:28:29	309	180	RMAN: incremental datafile backup: Set Count 137090: 940542 out of 2557440 Blocks done
512	55527	RMAN: aggregate input	backup	76514816	4360436	Blocks	01.08.2014 12:28:28	2085	126	RMAN: aggregate input: backup 33: 4360436 out of 76514816 Blocks done
281	56085	RMAN: incremental datafile backup	Set Count	3251200	969214	Blocks	01.08.2014 12:28:28	426	181	RMAN: incremental datafile backup: Set Count 137086: 969214 out of 3251200 Blocks done

Let's join V$SESSION_LONGOPS with V$SESSION and (if desirable) V$PROCESS, to view only the detailed rows, and estimate the progress in percent. This query is very useful to quickly get an overview of your backup, whether or not it is progressing according to expected speed etc:

SELECT S.CLIENT_INFO "Client Info", SL.OPNAME "Operation" ,SL.MESSAGE, SL.SID, SL.SERIAL#, P.SPID "OS Process ID", SL.SOFAR "So Far", SL.TOTALWORK "Totalwork", ROUND(SL.SOFAR/SL.TOTALWORK*100,2) "% complete"
FROM V$SESSION_LONGOPS SL INNER JOIN V$SESSION S ON SL.SID = S.SID 
                          INNER JOIN V$PROCESS P ON P.ADDR = S.PADDR
AND OPNAME LIKE 'RMAN%'
AND OPNAME NOT LIKE '%aggregate%'
AND TOTALWORK != 0
AND SOFAR <> TOTALWORK
/

Client Info	Operation	MESSAGE	SID	SERIAL#	OS Process ID	So Far	Totalwork	% complete
rman channel=tsm_channel_0	RMAN: incremental datafile backup	RMAN: incremental datafile backup: Set Count 137096: 1173118 out of 2252800 Blocks done	203	7409	42926864	1173118	2252800	52,07
rman channel=tsm_channel_1	RMAN: incremental datafile backup	RMAN: incremental datafile backup: Set Count 137095: 2029054 out of 2393600 Blocks done	281	56085	52101750	2029054	2393600	84,77
rman channel=tsm_channel_2	RMAN: incremental datafile backup	RMAN: incremental datafile backup: Set Count 137097: 293246 out of 2096640 Blocks done	357	31627	56361104	293246	2096640	13,99
rman channel=tsm_channel_3	RMAN: incremental datafile backup	RMAN: incremental datafile backup: Set Count 137099: 174206 out of 2048000 Blocks done	398	15079	52756692	174206	2048000	8,51
rman channel=tsm_channel_4	RMAN: incremental datafile backup	RMAN: incremental datafile backup: Set Count 137098: 460286 out of 2048000 Blocks done	439	17451	29032454	460286	2048000	22,47
rman channel=tsm_channel_5	RMAN: incremental datafile backup	RMAN: incremental datafile backup: Set Count 137100: 196094 out of 2048000 Blocks done	476	55467	9700724	196094	2048000	9,57

Notice how the V$RMAN_STATUS only hold information on aggregated level (2 rows, one for each operation although the first spawns the other):

SELECT SID,OPERATION,STATUS,MBYTES_PROCESSED, START_TIME, END_TIME, OBJECT_TYPE, OUTPUT_DEVICE_TYPE 
FROM V$RMAN_STATUS WHERE STATUS = 'RUNNING';

SID	OPERATION	STATUS	MBYTES_PROCESSED	START_TIME	OBJECT_TYPE	OUTPUT_DEVICE_TYPE
512	BACKUP	RUNNING	226641,953125	01.08.2014 12:28:28	DB INCR	SBT_TAPE
512	RMAN	RUNNING	0	01.08.2014 12:28:28