Error message 1555 when synching metadata in LRCC

Has anyone had a problem when synching metadata - specifically keywords - on LRCC .  I've started getting error code 1555 stating the location/name of the database, then UNIQUE constraint failed: AgLibraryKeywordImage.id_local sql: INSERT INTO AgLibraryKeywordImage( id_local, image, tag ) VALUES( ?, ?, ?); (error code 1555)

Any idea what that means?

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