diff --git a/erpnext/stock/doctype/serial_no/serial_no.py b/erpnext/stock/doctype/serial_no/serial_no.py
index 6042ed4..a2748d0 100644
--- a/erpnext/stock/doctype/serial_no/serial_no.py
+++ b/erpnext/stock/doctype/serial_no/serial_no.py
@@ -846,16 +846,15 @@
 
 	pos_transacted_sr_nos = query.run(as_dict=True)
 
-	reserved_sr_nos = []
-	returned_sr_nos = []
+	reserved_sr_nos = set()
+	returned_sr_nos = set()
 	for d in pos_transacted_sr_nos:
 		if d.is_return == 0:
-			reserved_sr_nos += get_serial_nos(d.serial_no)
+			[reserved_sr_nos.add(x) for x in get_serial_nos(d.serial_no)]
 		elif d.is_return == 1:
-			returned_sr_nos += get_serial_nos(d.serial_no)
+			[returned_sr_nos.add(x) for x in get_serial_nos(d.serial_no)]
 
-	for sr_no in returned_sr_nos:
-		reserved_sr_nos.remove(sr_no)
+	reserved_sr_nos = list(reserved_sr_nos - returned_sr_nos)
 
 	return reserved_sr_nos