check for existing task before appending to child table (#15183)

erpnext/projects/doctype/task/task.py

@@ -141,6 +141,7 @@ class Task(NestedSet):
 	def populate_depends_on(self):
 		if self.parent_task:
 			parent = frappe.get_doc('Task', self.parent_task)
+			if not self.name in [row.task for row in parent.depends_on]:
 				parent.append("depends_on", {
 					"doctype": "Task Depends On",
 					"task": self.name,