Find every copy
Search your inbox and help desk for the customer's address and the subject. Note who received it, who replied and what was said. You need the full picture before you answer.
Pick one owner
Decide who will reply, usually the person who owns the topic or the first to respond. Tell the others in your shared channel so nobody sends a second answer.
Merge the threads
Join the copies into one ticket so the history sits in one place. The next person who opens it should see every message the customer sent.
Reply once, for everyone
Answer the question in one message and say it comes from the whole team. If an earlier reply was wrong or different, correct it plainly without blaming a colleague.
Explain the single route
Tell the customer where to write next time and that one thread gets the fastest answer. Do it kindly. They emailed everyone because they felt unheard, so show them that one reply is enough.
Fix the cause
Ask why they went wide. A slow first reply, a missed message or an unclear contact page are common causes. Fix that one and the habit usually fades.