Quote from RFC1930, "Guidelines for creation, selection, and registration of an Autonomous System (AS)", page 7:
"With the introduction of aggregation it should be noted that a prefix may be represented as residing in more than one AS, however, this is very much the exception rather than the rule".
Quote from RFC2725, "Routing Policy System Security", page 9:
"Route objects may exist for the same prefix with multiple origin AS values due to common multihoming practice that does not require a unique origin AS".
RFC1930, Category: Best Current Practice, March 1996 RFC2725, Category: Standards Track, Dec 1999
Just a point of clarification: that some text in a standards track RFC mentions a practice and permits that practice to be described, does not mean that the practice itself is standardized or recommended. But we all knew that already, right? Regards, - Håvard