photo:addkeyword why catalog:createkeyword is exists

I'm trying to understand the addkeyword, when renaming my images I need to preserve the keywords, label and caption.

It works, but I endup with double keywords (if they are embedded in the file), and why the need of cataloging:createkeyword()  if the keyword already exists in Lightroom.

So it seems that I need to create a function to compare the old keywords versus the renamed file.

--- Set metadata back ----

photo:addKeyword(keywordTags )

gives an error -> assertion failed!

So first

cataloging:createkeyword() 

photo:addKeyword(keywordTags )