diff --git a/frappe/database/schema.py b/frappe/database/schema.py
index 5920d14c3d..9e689f8c3c 100644
--- a/frappe/database/schema.py
+++ b/frappe/database/schema.py
@@ -17,18 +17,18 @@ class DBTable:
 		self.doctype = doctype
 		self.table_name = f"tab{doctype}"
 		self.meta = meta or frappe.get_meta(doctype, False)
-		self.columns = {}
+		self.columns: dict[str, DbColumn] = {}
 		self.current_columns = {}
 
 		# lists for change
-		self.add_column = []
-		self.change_type = []
-		self.change_name = []
-		self.add_unique = []
-		self.add_index = []
-		self.drop_unique = []
-		self.drop_index = []
-		self.set_default = []
+		self.add_column: list[DbColumn] = []
+		self.change_type: list[DbColumn] = []
+		self.change_name: list[DbColumn] = []
+		self.add_unique: list[DbColumn] = []
+		self.add_index: list[DbColumn] = []
+		self.drop_unique: list[DbColumn] = []
+		self.drop_index: list[DbColumn] = []
+		self.set_default: list[DbColumn] = []
 
 		# load
 		self.get_columns_from_docfields()
diff --git a/frappe/tests/test_db_update.py b/frappe/tests/test_db_update.py
index bbb10aec2c..0cc0de065c 100644
--- a/frappe/tests/test_db_update.py
+++ b/frappe/tests/test_db_update.py
@@ -1,4 +1,5 @@
 import frappe
+from frappe.core.doctype.doctype.test_doctype import new_doctype
 from frappe.core.utils import find
 from frappe.custom.doctype.property_setter.property_setter import make_property_setter
 from frappe.tests.utils import FrappeTestCase
@@ -87,6 +88,36 @@ class TestDBUpdate(FrappeTestCase):
 		email_sig_column = get_table_column("User", "email_signature")
 		self.assertEqual(email_sig_column.index, 1)
 
+	def check_unique_indexes(self, doctype: str, field: str):
+		indexes = frappe.db.sql(
+			f"""show index from `tab{doctype}` where column_name = '{field}' and Non_unique = 0""",
+			as_dict=1,
+		)
+		self.assertEqual(
+			len(indexes), 1, msg=f"There should be 1 index on {doctype}.{field}, found {indexes}"
+		)
+
+	def test_unique_index_on_install(self):
+		"""Only one unique index should be added"""
+		for dt in frappe.get_all("DocType", {"is_virtual": 0, "issingle": 0}, pluck="name"):
+			doctype = frappe.get_meta(dt)
+			fields = doctype.get("fields", filters={"unique": 1})
+			for field in fields:
+				with self.subTest(f"Checking index {doctype.name} - {field.fieldname}"):
+					self.check_unique_indexes(doctype.name, field.fieldname)
+
+	def test_unique_index_on_alter(self):
+		"""Only one unique index should be added"""
+
+		doctype = new_doctype(unique=1).insert()
+		field = "some_fieldname"
+
+		doctype.fields[0].length = 142
+		doctype.save()
+
+		doctype.delete()
+		self.check_unique_indexes(doctype.name, field)
+
 
 def get_fieldtype_from_def(field_def):
 	fieldtuple = frappe.db.type_map.get(field_def.fieldtype, ("", 0))