SQL Standard – Where to Find the SQL Standard Document

I think it's an implementation detail.

A conforming implementation is not required to perform the exact sequence of actions defined in the General Rules, provided its effect on SQL-data and schemas, on host parameters and host variable, and on SQL parameters and SQL variables is identical to the effect of that sequence. The term effectively is used to emphasize actions whose effect might be achieved in other ways by an implementation.¹

I think an implementer could evaluate a common table expression 20 times, even in 20 different ways, and still have a conforming implementation. The only relevant issue is whether "its effect . . . is identical to the effect" of the sequence of actions defined in the General rules.

[1]. Section 6.3.3.3, "Rule evaluation order", in a draft of the SQL 2008 standard, having the local filename 5CD2-01-Framework-2006-01.pdf, p. 41 I have no idea where I got it. Google might know.

The wiscorp.com page SQL Standards has several older revisions and drafts of SQL:20nn (zip):

The 7IWD2-02-Foundation-2011-12.pdf, with a date of 2011-12-21 has at page 289:

If at least one of S1 and S2 is the null value, then the result of the <concatenation> is the null value.