Documentation for SORT

[John Darrington]

The documentation for sort says.

 The sort algorithms used by @cmd{SORT CASES} are stable.  That is,
 records that have equal values of the sort variables will have the
 same relative order before and after sorting.  As a special case,
 re-sorting an already sorted file will not affect the ordering of
 cases.

The last sentence confuses me.   Why is it a special case?  Isn't that
property necessary for a stable sort?

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