问在alter_column之后创建索引时出现Django south错误(字段更改)

EN

class HardwareModel(models.Model):
    name = models.CharField(max_length=128,unique=True)
    series =models.CharField(max_length=128)
    def __unicode__(self):
        return "%s" % (self.name)

class HardwareModel(models.Model):
    name = models.CharField(max_length=128,unique=True)
    series = models.ForeignKey(HardwareModelSeries, blank=True)
    def __unicode__(self):
        return "%s" % (self.name)

    # Renaming column for 'HardwareModel.series' to match new field type.
    db.rename_column('iamictinfra_hardwaremodel', 'series', 'series_id')
    # Changing field 'HardwareModel.series'
    db.alter_column('iamictinfra_hardwaremodel', 'series_id', self.gf('django.db.models.fields.related.ForeignKey')(to=orm['iamictinfra.HardwareModelSeries']))
    # Adding index on 'HardwareModel', fields ['series']
    db.create_index('iamictinfra_hardwaremodel', ['series_id'])

south DEBUG execute "CREATE INDEX "iamictinfra_hardwaremodel_b18a98d" ON "iamictinfra_hardwaremodel" ("series_id");" with params "[]"
FATAL ERROR - The following SQL query failed: CREATE INDEX "iamictinfra_hardwaremodel_b18a98d" ON "iamictinfra_hardwaremodel" ("series_id");
The error was: index iamictinfra_hardwaremodel_b18a98d already exists