The unique constraint was created here
<changeSet id="20110708-2105" author="cta" dbms="mysql">
<preConditions onFail="MARK_RAN">
<not>
<indexExists indexName="concept_source_unique_hl7_codes" />
</not>
</preConditions>
<comment>
Add unique constraint to the concept_source table
</comment>
<addUniqueConstraint tableName="concept_source" columnNames="hl7_code" constraintName="concept_source_unique_hl7_codes"/>
</changeSet>
And the table renamed later
<changeSet id="20110301-1030a" author="wyclif" dbms="mysql">
<preConditions onFail="MARK_RAN">
<and>
<not><tableExists tableName="concept_reference_source" /></not>
</and>
</preConditions>
<comment>Renaming the concept_source table to concept_reference_source</comment>
<renameTable oldTableName="concept_source" newTableName="concept_reference_source"/>
</changeSet>
But AFAIK null values do not count for uniqueness. If they contain spaces, try setting them to null before updating.
Cheers