Currently, keyword synonyms get exported as top-level keywords; if the exported image is reimported into the catalog, those synonyms end up getting added to the catalog as top-level keywords.
Suggestion: If the export option Write Keywords As Lightroom Hierarchy is selected, then export the synonyms hierarchically, so that if they are reimported into the catalog, they get imported as synonyms under their existing parent keyword.
For example, suppose there is the keyword Family > John with synonym Jack. These would get exported into the metadata field XMP:HierarchicalSubject as:
When reimported, LR would recognize that Family|John|Jack is a synonym of Family|John in the catalog and not create a new keyword.
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Good idea. For the mean time, ExportMaster assures exported keywords will be *exactly* the same as source master. At some point, I will probably move ExportMaster functionality to Exportant, but as things stand today, one should use post-process action from one or the other, but not both simultaneously.