This message indicates that two mapping tables have the same name, and one of the duplicate mapping tables needs to be removed. However, formatting errors in the mapping file may cause the MTA to wrongly interpret something as a mapping table name. For example, failure to properly indent a mapping table entry will cause the MTA to think that the left hand side of the entry is actually a mapping table name. Check your mapping file for general form and check the mapping table names.
A blank line should precede and follow any line with a mapping table name. However, no blank lines should be interspersed among the entries of a mapping table.