MailMergeOptions.merge_duplicate_regions property. Gets or sets a value indicating whether all of the document mail merge regions with the name of a data source should be merged while executing of a mail merge with regions against the data source or just the first one....MailMergeOptions.merge_duplicate_regions property. Gets or sets a value indicating whether all of the document mail merge regions with the name of a data source should be merged while executing of a mail merge with regions against the data source or just the first one.