The moment a shop puts a second person on a second device, a reasonable worry appears: if both are billing, and the connection is unreliable, what stops the same sale being counted twice — or being lost entirely?
How it works, briefly
A bill is written to the device first and considered done there. That is why billing finishes at the same speed with no signal as with a good one — nothing is waiting on a network round trip while a customer stands at the counter.
Syncing happens afterwards, in the background. Each bill carries an identity of its own, so when the connection is patchy and the app retries, the second attempt at an already-received bill is recognised and discarded rather than written as a new sale. That is the whole of the duplicate protection, and it matters most in exactly the conditions where you would never notice a problem: a busy hour on a bad connection.
What can still go wrong, and what it looks like
| What you see | What it usually is |
|---|---|
| The same sale appears twice, with two different bill numbers | Entered by hand twice, usually by two people who each thought the other had not done it. Not a sync problem. |
| Stock briefly negative on a fast-moving item | Two devices both sold the last few while offline. It resolves when both have synced. |
| A bill missing entirely | Almost always a bill never saved on the device — interrupted mid-entry — rather than one lost in transit. |
| Today's total lower than expected on one device | That device has not finished syncing. Check the other one is online before assuming money is missing. |
The first row is the one to know about, because it is the common case and it is not a software fault. Two people covering a counter, one asks whether the sale went in, the other assumes not, and it goes in twice. The fix is a habit rather than a setting: whoever takes the money enters the bill.
Running a second counter well
- 1
Own logins, always
Two people on one account is what makes every one of the situations above impossible to untangle, because no entry can be traced to a person.
- 2
Agree who enters what
Whoever takes the payment raises the bill. It sounds obvious and it is the single rule that prevents most double entries.
- 3
Let both devices finish syncing before closing the day
Two minutes on wifi. Closing from a device that has not received the other counter's bills produces a shortfall that does not exist.
- 4
Count the fastest-moving items more often
They are where simultaneous offline selling shows up first, and where a real difference is most likely to be hiding among the explainable ones.
The bill finishes on the phone in your hand. Everything after that is the software's problem, not the customer's.