diff --git a/erpnext/hr/doctype/salary_structure_assignment/salary_structure_assignment.py b/erpnext/hr/doctype/salary_structure_assignment/salary_structure_assignment.py
index d4e8ccd..2168a36 100644
--- a/erpnext/hr/doctype/salary_structure_assignment/salary_structure_assignment.py
+++ b/erpnext/hr/doctype/salary_structure_assignment/salary_structure_assignment.py
@@ -8,6 +8,8 @@
 from frappe.utils import getdate
 from frappe.model.document import Document
 
+class DuplicateAssignment(frappe.ValidationError): pass
+
 class SalaryStructureAssignment(Document):
 	def validate(self):
 		self.validate_dates()
@@ -56,7 +58,8 @@
 			})
 
 		if assignment:
-			frappe.throw(_("Active Salary Structure Assignment {0} found for employee {1} for the given dates").format(assignment[0][0], self.employee))
+			frappe.throw(_("Active Salary Structure Assignment {0} found for employee {1} for the given dates").
+				format(assignment[0][0], self.employee), DuplicateAssignment)
 
 def get_assigned_salary_structure(employee, on_date):
 	if not employee or not on_date:
diff --git a/erpnext/patches/v11_0/create_salary_structure_assignments.py b/erpnext/patches/v11_0/create_salary_structure_assignments.py
index 8648e15..12b477b 100644
--- a/erpnext/patches/v11_0/create_salary_structure_assignments.py
+++ b/erpnext/patches/v11_0/create_salary_structure_assignments.py
@@ -4,24 +4,28 @@
 from __future__ import unicode_literals
 import frappe
 from datetime import datetime
+from erpnext.hr.doctype.salary_structure_assignment.salary_structure_assignment import DuplicateAssignment
 
 def execute():
 	frappe.reload_doc("hr", "doctype", "salary_structure_assignment")
 	for d in frappe.db.sql("""
 		select sse.*, ss.company from `tabSalary Structure Employee` sse, `tabSalary Structure` ss
 		where ss.name = sse.parent AND sse.employee in (select name from `tabEmployee` where ifNull(status, '') != 'Left')""", as_dict=1):
-		s = frappe.new_doc("Salary Structure Assignment")
-		s.employee = d.employee
-		s.employee_name = d.employee_name
-		s.salary_structure = d.parent
-		s.from_date = d.from_date
-		s.to_date = d.to_date if isinstance(d.to_date, datetime) else None
-		s.base = d.base
-		s.variable = d.variable
-		s.company = d.company
+		try:
+			s = frappe.new_doc("Salary Structure Assignment")
+			s.employee = d.employee
+			s.employee_name = d.employee_name
+			s.salary_structure = d.parent
+			s.from_date = d.from_date
+			s.to_date = d.to_date if isinstance(d.to_date, datetime) else None
+			s.base = d.base
+			s.variable = d.variable
+			s.company = d.company
 
-		# to migrate the data of the old employees
-		s.flags.old_employee = True
-		s.save()
+			# to migrate the data of the old employees
+			s.flags.old_employee = True
+			s.save()
+		except DuplicateAssignment:
+			pass
 
 	frappe.db.sql("update `tabSalary Structure` set docstatus=1")
\ No newline at end of file