There have been a few BPotD entries where the plant’s scientific name is apparently duplicated. Does the duplication of an epithet (e.g., var. occidentale) add extra information? Yes! Without knowing anything else about this taxon, it gives two additional pieces of info: 1) this was (almost certainly) the plant first described by a taxonomist as representative of the species. Often, but by no means always, this also represents the most common biological form, based on the notion that the most common taxa within a species will often have been collected and described first; and 2) there is at least one other biological entity recognized at the same level. In the case of Cirsium occidentale, there are at least four varieties recognized.
The duplication of epithets occurs from a practical and use perspective when a taxonomist makes an evidence-based decision to split up a species into taxa below the rank of species, i.e., subspecies, varieties or forms (note: contemporary requirements include also that the evidence and rationale must be published in a peer-reviewed scientific journal). As soon as one of these infraspecific taxa is carved off from what was once a singular species, the remainder of the species gets tagged with the duplicate name to differentiate it from the new entity. I’ll give a hypothetical example. Say there was a species named Aa ee and someone came along and said, “This part of that species is different.”, I’ll publish it as Aa ee var. ii – it is now necessary that the remainder of the original grouping have a name that can be used to communicate about it separately, i.e., Aa ee var. ee. The entire species, including all of its varieties, can still be discussed by using Aa ee. For a shorter version of what I just wrote, see Article 25 of the International Code of Botanical Nomenclature.
For the purposes of this cobweb thistle, knowledge of the reasoning for the name is required to understand its distribution. The entire species Cirsium occidentale has a distribution that includes Oregon, California and (possibly) northwest Mexico. However, Cirsium occidentale var. occidentale is native only to California.
Photography resource link: To my mind, one of the best contemporary nature photographers rarely has nature in his images. He is one of the few whose every image provokes an emotional response in me: Edward Burtynsky.