Email forwarding - emails behaving strangely

Question

I have a user that we enabled the Email forwarding option to her, let's call this user (X) So X is on a leave and her emails are being forwarded to her colleague (Y).

Now when someone sends an email to X and Y at the same time. Y would see only one copy of that email in her mailbox, she wouldn't see two times the same email, she would actually see the one that sends to X, inside the folder that has a rule applied to it, that any emails send to X it would landed in some certain folder in Y's mailbox.

I have disabled the internal rule that was sending X's emails to some certain outlook folder but that doesn't change anything.

I am suspecting it is some kind of technology behind that to reduce the email duplication, but I cannot find any official document for that!
Any idea why this is happening that way?