I thought customers who send repeat messages regarding the same questions does not count against response rating? Also I was under the understanding it was first time messages not a repeat customer that sent a new message thread.
You state that it's a new message thread, not a continuation of an existing one. So, you need to respond or mark it as spam.
Already answered in your other thread: https://community.etsy.com/t5/Technical-Issues/Message-response/m-p/142359941#M779016
If it's a new message thread, it's likely a help request, which are always treated as separate threads. Both need replies.
each new message thread has to be answered,
they are highlighted in blue, so you can't miss them,
the ones with "help request" are separated, as that starts the 48 hour clock for a case to be opened, and if you don't respond to that in time, you have no seller protection