A customer sent me a duplicate message asking the same question; I only answered one, and the non-reply to the other one has negatively impacted my message reply rate, so I am no longer on track for star seller. Any way to have Etsy support fix this for me? Since I did technically reply to this customer.
Nope. You can send the duplicate to spam, but I wouldn't recommend that because that blocks them from contacting you again.
If you didn't reply to both of them because "didn't think I had to" - that's a mistake on your part. All messages (including duplicates) need to be responded to within 24 hours.
All Initial messages in a thread (for a particular account) must be responded to within 24 hours for the star seller badge.
If the person messaged you twice from the same account and you responded to the first message ... you should be ok from a star seller standpoint.
If you responded to the 2nd message ... star seller will ding you for not responding to the first message.
If the buyer is using multiple accounts, each counts as a unique thread.
You must respond to all messages or mark as spam.
Etsy won't fix this.
The rules have been clear for at least 3 maybe 4 years about replying to every new message.
If a shopper/buyer sends you 5 new messages you need to reply to everyone of them.
If you spam/report them then that person can no longer message you.
Even replying with a simple emoji counts as a reply! : )