Emails In Response To Autoresponders Going To Two Email Address's

Anyone here got any experience with autoresponders?

An autoresponder is setup, so that anyone emailing to admin@example.com, gets an autoresponder from enquiries@example.com asking for a couple of details, the person then replies to the autoresponder with the details, they then receive another autoresponder from enquiries@example.com confirming receipt of their last email. That's what should happen, but instead, the person keeps getting an autoresponder from enquiries@example.com asking for more details, even after they've already emailed the details?

If it's any help, when I've been testing this, when replying to the first autoresponder, I notice in the 'to' line it mentions both email address's, in the following format:
Enquiries@example.com <admin@example.com>