In AR Manual transaction, user receving notification mails

Hi,

There is an issue with AR, a manual transaction was created by user X. A notification mail saying like, transaction was created sent to user X and user Y. once its submitted by user X, the transaction submitted for approval notification mail sent to user X, user Y and its respective approver for that approval hierarchy.

The issues is, why user Y receiving notification mail when a manual transaction created by user X. There is no link between two user. And user Y core id is not at all there in approval hierarchy setup. We verified it, We connected with user X & user Y, the don't have any vacation rule in R12.