To show new order

[tan_chungkam]

I have a table as shown below:

Month	Order No.	Order Type
Jan	11111	New Order
Jan	22222	New Order
Jan	33333	New Order
Feb	44444	New Order
Feb	55555	New Order
Feb	22222	Existing Order
Feb	33333	Existing Order

I would like to using a function/formula to create the order type column.

If the order no. appear in previous month, then it is an existing order.

If the order no. do not appear in previous month, then it is a new order.

Is there any way to do it? Thanks for the help.