diff --git a/app/models.py b/app/models.py
index 9d0db0563..2b91d9395 100644
--- a/app/models.py
+++ b/app/models.py
@@ -579,6 +579,9 @@ class AnnualBilling(db.Model):
     UniqueConstraint('financial_year_start', 'service_id', name='ix_annual_billing_service_id')
     service = db.relationship(Service, backref=db.backref("annual_billing", uselist=True))
 
+    __table_args__ = (UniqueConstraint(
+        'service_id', 'financial_year_start', name='uix_service_id_financial_year_start'),)
+
     def serialize_free_sms_items(self):
         return {
             'free_sms_fragment_limit': self.free_sms_fragment_limit,
diff --git a/migrations/versions/0351_unique_key_annual_billing.py b/migrations/versions/0351_unique_key_annual_billing.py
new file mode 100644
index 000000000..288b18395
--- /dev/null
+++ b/migrations/versions/0351_unique_key_annual_billing.py
@@ -0,0 +1,34 @@
+"""
+
+Revision ID: 0351_unique_key_annual_billing
+Revises: 0350_update_rates
+Create Date: 2021-04-12 09:02:45.098875
+
+"""
+import os
+
+from alembic import op
+
+revision = '0351_unique_key_annual_billing'
+down_revision = '0350_update_rates'
+
+environment = os.environ['NOTIFY_ENVIRONMENT']
+
+
+def upgrade():
+    index = """
+        CREATE UNIQUE INDEX CONCURRENTLY uix_service_id_financial_year_start 
+        ON annual_billing (service_id, financial_year_start)
+    """
+    constraint = """
+        ALTER TABLE annual_BILLING add constraint uix_service_id_financial_year_start
+         UNIQUE USING INDEX uix_service_id_financial_year_start
+    """
+    op.execute('COMMIT')
+    op.execute(index)
+    op.execute(constraint)
+
+
+def downgrade():
+    op.drop_constraint('uix_service_id_financial_year_start', 'annual_billing', type_='unique')
+