If you're receiving duplicate tickets from eBay, the most likely cause is that your eBay account is set up to send notification emails to the mail registered to your Zendesk instance. Thus you have one ticket created for receiving an email message and another created by ChannelReply.
To identify tickets created by incoming email messages, look at the line under the ticket's subject. After the sender's email, you'll see something like "via %firstname.lastname@example.org%," specifying one of your registered Zendesk emails.
To avoid this error, we suggest setting up your eBay account as follows:
We also suggest that you create a trigger that will close tickets that come from eBay by email. To identify such tickets, you can create an organization named "eBay Users" consisting of the following email domains:
Then you can create a trigger that will close tickets from that organization. The trigger should contain two conditions: the ticket channel is email; the ticket organization is "eBay Users." The trigger action should be: close ticket.