[members-discuss] An ISP offers to announce our prefix. Is that normal?

I have a follow up question, regarding my impression that "it sounds 
broken (to me)" if more then one AS announces the same prefix as origin:

/* I knew I have read it somewhere... Maybe it's just old and dusted and 
did not survive the turn of the millennium */

https://tools.ietf.org/html/rfc1930#page-8 from 1996
 >7. One prefix, one origin AS

    Generally, a prefix can should belong to only one AS. This is a
    direct consequence of the fact that at each point in the Internet
    there can be exactly one routing policy for traffic destined to each
    prefix. In the case of an prefix which is used in neighbor peering
    between two ASes, a conscious decision should be made as to which AS
    this prefix actually resides in.

    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. This happens when
    aggregating using the AS_SET attribute in BGP, wherein the concept of
    origin is lost. In some cases the origin AS is lost altogether if
    there is a less specific aggregate announcement setting the
    ATOMIC_AGGREGATE attribute.

So in the second paragraph: "this is very much the exception rather than 
the rule" -- does it just turned around over time?

So today/in modern times: The same prefix, or the same specific 
network/subnet can be originated from more then one AS and things do not 
break? In this thread it was mentioned that even valid signed ROA can be 
created for this scenario.

But I'm still puzzled if it is only me that validates it, or do both 
(the ISP and myself) do validate it?

Thanks again for your time and help!
Bernd