An item that already exist in the list

Question

if someone insert an item that already exist in the list- I want that an email will send to the manager.
can I do this? how?

Answer 1

I will write code that send an email, but my question is in which event?
ItemUpdating event will not help me because the item will not update if the item already exist in the list...
thank you!
in item adding get the column value of the item which is getting added
string value= properties.AfterProperties["ColumnName"].ToString(); OR string name= web.Lists[properties.ListId].Fields["Column Display Name"].InternalName;string value= properties.AfterProperties[name].ToString();
after getting the value search if the item exists which has the same field value. 
you can iterate the list for searching but i will recommend you to use CAML query