Duplicate changeSet not failing as expected

So I have an Oracle database which I am updating...  in the process of testing my DB build system, I created a new change which is identical in content to an older change with a different ID.  Both are adding a column with the same name, using SQL.  I would expect the 2nd to fail, since it would attempt to create a column which already exists, but the liquibase command updateTestingRollback reports:

Liquibase 'updateTestingRollback' Successful

Any ideas what the potential cause may be?

<changeSet id="4" author="anonymous" failOnError="true">
  <comment> - </comment>
  <sql>ALTER TABLE person ADD (col_a VARCHAR(1));</sql>
  <rollback>ALTER TABLE person DROP COLUMN col_a;</rollback>
</changeSet>

<changeSet id="5" author="anonymous" failOnError="true">
  <comment> - </comment>
  <sql>ALTER TABLE person ADD (col_a VARCHAR(1));</sql>
  <rollback>ALTER TABLE person DROP COLUMN col_a;</rollback>
</changeSet>