Duplicate replies happen in the window between opening an email and sending the answer, because nothing in a plain mailbox shows that someone is already writing. Four things close that window: one named owner per conversation, visible in the mailbox itself; a signal that someone is currently replying; a done state that is not “read”; and internal notes stored next to the message rather than in a chat app. Flags, personal subfolders and a quick shout across the room all fail for the same reason, which is that they depend on somebody remembering.
Why two people answer the same email
A customer writes to contact@ on Friday evening asking for a price. Monday at 9:05, Julie opens it and starts writing. At 9:07 Thomas opens the same mailbox, sees the message, and cannot tell that anyone is on it. He starts writing too. At 9:15 Julie sends. At 9:17 Thomas sends.
The customer now has two replies. If they are identical, your team looks disorganised. If Julie offered a 10% discount and Thomas quoted full price, the customer will hold you to the cheaper one, and reasonably so.
The failure is not carelessness. It is structural. A mailbox shows you what has been sent, never what is being written. Everything happens in the gap between the two, and that gap is exactly as long as it takes to write a careful reply, which for anything commercial is ten to twenty minutes. Multiply by the number of people who open the mailbox each morning and collisions become a matter of arithmetic, not attitude.
The read status lies to you, in both directions
Most teams believe the read status protects them. It does the opposite, and it fails twice.
It creates false negatives. Julie opened the message, so it shows as read for everyone. Thomas concludes she is handling it and moves on. But Julie only opened it to check a reference before a phone call, got pulled into that call, and never came back. The email is marked read, nobody is on it, and the customer waits three days. This is the failure mode that never gets counted, because nothing visibly goes wrong.
It creates false positives. In the other direction, an unread message tells you nothing either. Thomas may have read it on his phone in the car park, decided to answer properly from his desk, and marked it unread deliberately. It looks free. It is not.
Read and unread were designed for one person managing their own mail. They carry no information about intent, and intent is the only thing that matters when several people share an address. A message someone opened is not a message someone is handling.
What Microsoft and Google actually give you
Worth knowing before you build a process on top of the platform, because the platforms are more honest about their limits than most people realise.
Microsoft documents duplicates as a known symptom. A Microsoft 365 shared mailbox supports a maximum of 25 users. Beyond that, Microsoft warns that people accessing it at the same time might experience connection failures or duplicated messages. Note what that means: past a certain size, you get duplicates from the platform itself, on top of the human ones. Microsoft’s own recommendation at that point is to move to a Microsoft 365 group.
Google shipped a duplicate button, which tells you how common this is. A Google Group with Collaborative Inbox enabled lets members take a conversation, assign it to someone, and mark it complete, needing no further action, or a duplicate of another conversation. Two caveats: conversation history has to be turned on for any of it to work, and assigning requires the “Who can moderate metadata” permission, which an owner or manager grants. Marking something a duplicate also locks the conversation.
Gmail delegation splits the address. Up to 1,000 delegates on a work or school account, though Google notes that with typical use, 40 can access a mailbox at the same time. It solves the password problem. It does not solve this one: two delegates can write at once with no indication, and when a delegate sends, their own email address appears rather than the shared one.
None of the three tells you, in the moment, that a colleague is typing. That is the specific gap.
The workarounds teams invent, and where each breaks
Flags and colour categories. Julie flags red, Thomas flags blue. It works until the one morning somebody is in a hurry and skips it, which is the morning it matters. A convention that has to be remembered under pressure will be forgotten under pressure.
Personal subfolders. “Julie” and “Thomas” folders. This one is worse than doing nothing, because the message leaves the inbox. When Julie is off sick, her emails are invisible to everyone else, filed under a name rather than a status. You have built silos inside a mailbox whose entire purpose was to remove them.
Calling it out loud, or on Slack. “I’m taking the Martin one.” Works in a room of three where everyone is present and listening. Fails when one person is on a call, remote, or focused. Also fails structurally: the information about who owns a message lives in a different application from the message. Anyone who joins the team next year cannot reconstruct any of it.
One person owns the mailbox and forwards. Genuinely effective against duplicates, and it creates a bottleneck instead. That person spends the day sorting rather than answering, and everyone else gets copies landing in personal mailboxes, so replies scatter again. It also stops dead the day they go on holiday.
Time slots. Julie takes mornings, Thomas afternoons. The cleanest of the manual fixes, and worth doing if you cannot change tools this quarter. It breaks on anything that spans the handover, which is most conversations that matter.
The common thread: every one of these depends on a human remembering to signal something, in a place separate from where the work happens.
The four things that actually work
Whatever tool you use, these are the properties to look for.
One named owner per conversation, visible in the list. Not a folder, not a flag: a name attached to the message, that everyone sees without opening anything. The test is whether a colleague scanning the inbox for five seconds can tell who owns what.
A signal while someone is writing. This is the only mechanism that closes the actual gap, because it fires during the ten minutes when the damage happens rather than after. If the tool can show “Julie is replying” the moment she starts typing, the collision cannot occur.
A done state that is not “read”. An explicit closed or resolved status, separate from read, so the inbox tells you what is left rather than what has been glanced at.
Internal notes attached to the message. So “the customer already called about this, I quoted 450” lives on the conversation, where the next person will find it, instead of in a chat thread they were not part of.
What to set up this week
If you are not changing tools right now, do these in order. They cost nothing.
- Write down who answers what, by topic rather than by time. Quotes to Julie, scheduling to Thomas. Topic beats time slots because it survives handovers.
- Agree that opening a message means claiming it. If you open and do not answer, you say so.
- Pick one place, and only one, where claims are announced. Not Slack for some and out loud for others.
- Check the sent folder before replying to anything older than an hour. Clumsy, but it catches the expensive cases.
That is a floor, not a fix. It will reduce duplicates and it will keep costing attention every day, because it runs on discipline.
Where a shared inbox changes the arithmetic
A shared inbox provides the four properties above as behaviour rather than convention. With Trupeo, you connect the mailbox you already have, Gmail, Outlook, Microsoft 365 or any IMAP provider, keeping your address. Each person signs in with their own account. Each conversation gets one owner, an open or done status, tags and internal notes. When somebody starts writing, everyone else sees it before they open the message.
The difference is not that the team becomes more disciplined. It is that a collision stops requiring anyone to remember anything. See the features or our pricing, or read our complete shared inbox guide if you are weighing up the whole subject. We also compared this directly to the Gmail setup in Trupeo vs a Gmail shared mailbox, and covered the underlying trade-off between broadcast and ownership in shared mailbox vs distribution list.
Frequently asked questions
How do teams avoid duplicate replies in a shared inbox?
By making ownership visible before the reply is sent rather than after. In practice that means assigning each conversation to one person, showing in real time when somebody is writing, and using an explicit done status instead of the read flag. Teams that rely on flags, personal folders or verbal coordination still collide, because those depend on someone remembering to signal.
Why do two people reply to the same email?
Because a mailbox only shows sent messages, never messages being written. Between opening an email and sending the answer there is a window of ten to twenty minutes during which the message looks untouched to everyone else. Anyone who opens the mailbox in that window has no way to know it is already being handled.
Does marking an email as read stop duplicates?
No, and it causes a second problem. Read only means somebody opened it, not that they are answering, so a message read and abandoned looks handled and gets no reply at all. Read and unread were designed for one person managing their own mail and carry no information about intent.
Can Outlook or Gmail show when a colleague is replying?
Neither shows that a colleague is currently writing a reply in a shared mailbox. Microsoft 365 shared mailboxes and Gmail delegation both let several people work in one mailbox, but the signal that someone is typing right now is not part of either. Google’s Collaborative Inbox adds assignment and a duplicate status after the fact, not a live indication.
How many people can share one mailbox before it becomes a problem?
Microsoft documents a maximum of 25 users on a shared mailbox, and warns of connection failures or duplicated messages beyond that. In practice collisions start much earlier: from three people answering the same address daily, the window between opening and sending is enough to produce them regularly.
Sources:
- About shared mailboxes in Microsoft 365: the 25-user maximum for a shared mailbox, and Microsoft’s warning that concurrent access beyond it can cause connection failures or duplicated messages.
- Use a group as a Collaborative Inbox: the take, assign, mark complete and mark duplicate features, the permissions required to use them, and the locking of a conversation marked as a duplicate.
- Make a group a Collaborative Inbox: the requirement to enable conversation history before Collaborative Inbox features work.
- Delegate and collaborate on email: the delegate limits for work and personal accounts, the note that 40 delegates can access a mailbox at the same time with typical use, and the fact that a delegate’s own address appears when they send.
- Compare types of groups in Microsoft 365: the distinction between distribution groups, shared mailboxes and Microsoft 365 groups, and Microsoft’s stated purpose for each.