Two handsets with the same IMEI

Not possible, and that is the point — one reference is one physical device.

I do not know the IMEI yet

Use a shop reference for now and correct it later. An unlabelled handset on a shelf is the thing this is meant to prevent.

Put a second-hand device on the shelf with its own IMEI, condition, cost and price, so it is priced as itself rather than as a model.

By the end of this the handset is on the shelf as its own row, sellable by IMEI, with a cost that will show up correctly in profit when it sells.

Find or create the model
Inventory → Items. If you already stock that model, open it; if not, add it as a device. The model holds the name and the default price; the handsets hold everything that varies.
app.slickcell.com/inventory
Add the unit
Enter the IMEI or serial, the condition, the storage and colour, what you paid and what you are asking. Anything else the shop records — battery health, network, whether it is boxed — goes on the unit too.
The reference is free text. A full IMEI, a serial, the last five digits or your own sticker number are all fine; nothing is rejected for the wrong shape.
Check the model's quantity
It has gone up by one, because it counts the units. There is no number to correct.

Two handsets with the same IMEI: Not possible, and that is the point — one reference is one physical device.
I do not know the IMEI yet: Use a shop reference for now and correct it later. An unlabelled handset on a shelf is the thing this is meant to prevent.