Typically, KEEP will create multiple backup sets. Therefore, it would seem that one is for the SPFILE, one is for the archivelogs, one is for the controlfile and the additional one would be for any datafiles and since you aren't backing up any datafiles, it produces a backupset with just another backup of the spfile.
To create only one backupset, you could issue the following instead:
Code:
backup spfile keep until time 'sysdate+7' nologs tag='D_SP_TEST';
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