Reply to form email sends duplicate to InBox

We have 3 sites up using Gravity Forms, but the most recent site (which was originally a clone of site #2) seems to be creating a duplicate email or a referral loop of some kind. When my client replies to an email from the site's GF form it sends a new copy of the reply back to their InBox. So every time they reply a new email appears with their reply in it.

I've checked the email and it only contains the sender's email (being replied to) in the To field and nothing in a CC. I looked at the header as well and didn't see anything there.

This site is the primary domain for the hosting account.

The email used for the forms from the site is different than their primary email address. (i.e. the email GF uses is wordpress@domain.com while the client uses mycompany@domain.com).

This is the only one of the 3 sites that does this, not the other two. Again, this was originally a cloned site.

They are using Mac's email software for their email.

The email accounts have the MX through Google Apps, again same for all 3 sites.

ANY thoughts would be greatly appreciated.

I realize that it may be out of scope, but it is interesting to me that the duplication problem is only coming from form entries, and only from 1 of the 3 sites using Gravity Forms.

The form entry notification send the email from the email address of the person who submitted the form. I've had the client forward me emails and I've looked at them on their computers, and there is only one email in the From, no CC no BCC, and that email is the form entrant's. Looking at the headers of the email, it doesn't make sense.

Any ideas would be greatly appreciated. Thanks.