As it turns out the s3ftp used to mount the s3 bucket to our ftp server puts the file on s3 more than once. So the SNS topic is triggered more than once.

We need to deal with this, it's ok when updating a notification status from delivered to delivered. But the DailySortedLetter counts are being doubled.
Adding the file_name to the table as a unique key to get around this issue. It will mean we have multiple rows for each billing_day, but that's ok we can aggregate that.
This will also give us a way to see which file created which count.

migrations/versions/0178_add_filename.py

@@ -0,0 +1,29 @@
+"""
+
+Revision ID: 0178_add_filename
+Revises: 0177_add_virus_scan_statuses
+Create Date: 2018-03-14 16:15:01.886998
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+revision = '0178_add_filename'
+down_revision = '0177_add_virus_scan_statuses'
+
+
+def upgrade():
+    op.add_column('daily_sorted_letter', sa.Column('file_name', sa.String(), nullable=True))
+    op.create_index(op.f('ix_daily_sorted_letter_file_name'), 'daily_sorted_letter', ['file_name'], unique=False)
+    op.create_unique_constraint('uix_file_name_billing_day', 'daily_sorted_letter', ['file_name', 'billing_day'])
+    op.drop_index('ix_daily_sorted_letter_billing_day', table_name='daily_sorted_letter')
+    op.create_index(op.f('ix_daily_sorted_letter_billing_day'), 'daily_sorted_letter', ['billing_day'], unique=False)
+
+
+def downgrade():
+    op.drop_index(op.f('ix_daily_sorted_letter_billing_day'), table_name='daily_sorted_letter')
+    op.create_index('ix_daily_sorted_letter_billing_day', 'daily_sorted_letter', ['billing_day'], unique=True)
+    op.drop_constraint('uix_file_name_billing_day', 'daily_sorted_letter', type_='unique')
+    op.drop_index(op.f('ix_daily_sorted_letter_file_name'), table_name='daily_sorted_letter')
+    op.drop_column('daily_sorted_letter', 'file_name')