On the exporting window, I select the option "Write Keywords as Lightroom Hierarchy" ... and I export. After exporting and adding a file to my catalog, synonym keywords are added to my keywords list as different keywords. ... is there any cure for this behavior, other than unchecking "export synonyms" check box?
Unfortunately, your only options are to uncheck Export Synonyms for every keyword or to stop using synonyms. Overall, LR's handling of synonyms isn't well-thought-out, in my opinion. Please add your vote and detailed opinion to this feature request in the official Adobe feedback forum: Lightroom: Better handling of exporting and reimporting keyword synonyms | Photoshop Family Customer Community In general, a vote, and more importantly, details about why you want a feature, do influence Adobe. But unfortunately, the Library and keywords in particular have received no love from Adobe for many years, and not that many people have complained compared to other issues, so I'm not optimistic that anything would change before the next ice age. But we can always hope.
You could also ask the author of the Metadata Wrangler plugin for a feature that strips synonyms, or may strips all top-level keywords (which could be easier to implement). The author tends to much more responsive than Adobe.
Thanks John, I voted.