diff --git a/atst/domain/task_orders.py b/atst/domain/task_orders.py
index 24acdee7..ce4ba77b 100644
--- a/atst/domain/task_orders.py
+++ b/atst/domain/task_orders.py
@@ -1,9 +1,11 @@
 import datetime
+from sqlalchemy.exc import IntegrityError
 
 from atst.database import db
 from atst.models.clin import CLIN
 from atst.models.task_order import TaskOrder, SORT_ORDERING
 from . import BaseDomainClass
+from .exceptions import AlreadyExistsError
 
 
 class TaskOrders(BaseDomainClass):
@@ -12,9 +14,12 @@ class TaskOrders(BaseDomainClass):
 
     @classmethod
     def create(cls, portfolio_id, number, clins, pdf):
-        task_order = TaskOrder(portfolio_id=portfolio_id, number=number, pdf=pdf)
-        db.session.add(task_order)
-        db.session.commit()
+        try:
+            task_order = TaskOrder(portfolio_id=portfolio_id, number=number, pdf=pdf)
+            db.session.add(task_order)
+            db.session.commit()
+        except IntegrityError:
+            raise AlreadyExistsError("task_order")
 
         TaskOrders.create_clins(task_order.id, clins)
 
@@ -35,7 +40,11 @@ class TaskOrders(BaseDomainClass):
             task_order.number = number
             db.session.add(task_order)
 
-        db.session.commit()
+        try:
+            db.session.commit()
+        except IntegrityError:
+            raise AlreadyExistsError("task_order")
+
         return task_order
 
     @classmethod
diff --git a/tests/domain/test_task_orders.py b/tests/domain/test_task_orders.py
index 7bc4cf41..49b9bae6 100644
--- a/tests/domain/test_task_orders.py
+++ b/tests/domain/test_task_orders.py
@@ -2,6 +2,7 @@ import pytest
 from datetime import date, timedelta
 from decimal import Decimal
 
+from atst.domain.exceptions import AlreadyExistsError
 from atst.domain.task_orders import TaskOrders
 from atst.models import Attachment
 from atst.models.task_order import TaskOrder, SORT_ORDERING, Status
@@ -154,3 +155,18 @@ def test_task_order_sort_by_status():
     assert len(sorted_by_status["Expired"]) == 2
     assert len(sorted_by_status["Unsigned"]) == 1
     assert list(sorted_by_status.keys()) == [status.value for status in SORT_ORDERING]
+
+
+def test_create_enforces_unique_number():
+    portfolio = PortfolioFactory.create()
+    number = "1234567890123"
+    assert TaskOrders.create(portfolio.id, number, [], None)
+    with pytest.raises(AlreadyExistsError):
+        TaskOrders.create(portfolio.id, number, [], None)
+
+
+def test_update_enforces_unique_number():
+    task_order = TaskOrderFactory.create()
+    dupe_task_order = TaskOrderFactory.create()
+    with pytest.raises(AlreadyExistsError):
+        TaskOrders.update(dupe_task_order.id, task_order.number, [], None)