React Leaflet : ugly Warning: Each child in a list should have a unique "key" prop

Question

I've set an leaflet map component that iterate through users fetched from db that contains, for each of them, coordinates. Devtool throw me:

Warning: Each child in a list should have a unique "key" prop.

Not the first time I got this warning and I usually find easily how to fix it but this time I can't find a solution.

All is working fine but this warning keeps triggering me

Console Warning:

I've tried to put the key as Marker component identifier, Popup component identifier, both of them.

I've seen a post that recommend to put the key as Fragment identifier for each element of my map function. I tried in a div component (seems equal to Fragment to me?) still not working.

<MapContainer  id={!isMobile ? "browser-leaflet" : "mobile-leaflet"} center={POSITION} zoom={15} scrollWheelZoom={true}>
    <TileLayer
    attribution='&copy; <a href="https://www.openstreetmap.org/copyright">OpenStreetMap</a> contributors'
    url="https://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png?lang=fr"
    />
    {contacts.map(contact => (
        <div key={contact.email}> <-- last try here
            <Marker //also tried here// position={[contact.lat, contact.lon]} icon={new Icon({iconUrl: contact.is_client ? markerIconClient : markerIconProspect, iconSize: [25, 41], iconAnchor: [12, 41]})}>
                <Popup //also tried here// offset={[0, -15]}>
                    <p style={{marginBottom: "1vh", textAlign: "center", fontSize: "1.5em"}}>
                        {contact.raison_sociale}
                    </p>
                    <Button variant="sub" className="mx-auto" style={{border: "teal", borderRadius: "15px", fontSize: "1em", display: "flex", justifyContent: "center"}}>
                        <a 
                        href={"https://www.waze.com/fr/live-map/directions?navigate=yes&to=ll." + contact.lat + "%2C" + contact.lon}
                        style={{textDecoration: "none", color: "black", backgroundColor: "#D0FCB3"}}
                        >
                            Allez-y
                        </a>
                    </Button>
                </Popup>
            </Marker>
        </div>
    ))}
    <SetMap />
</MapContainer>

Error seems to be located @Geolocalisation.jsx (my whole page compo) line:102 which is where I set style and navbar based on client device.

return ( <--- here l:102
        <>
            {!isMobile ? (
                [
                    <BrowserSidenav />, 
                    <style>
                        {browserLeafletContainer}
                    </style>
                ]
            ) : (
                <style>
                    {mobileLeafletContainer}
                </style>
            )}

Does someone knows how to fix it? (P.S: sorry for bad English, not native and not using online translator)

Whole return from my component Geolocalisation.jsx : (funcs are just a users fetch, a useEffect that delete users with no lat/lon.

return (
        <>
            {!isMobile ? (
                [
                    <BrowserSidenav />, 
                    <style>
                        {browserLeafletContainer}
                    </style>
                ]
            ) : (
                <style>
                    {mobileLeafletContainer}
                </style>
            )}

            <MapContainer  id={!isMobile ? "browser-leaflet" : "mobile-leaflet"} center={POSITION} zoom={15} scrollWheelZoom={true}>
                <TileLayer
                attribution='&copy; <a href="https://www.openstreetmap.org/copyright">OpenStreetMap</a> contributors'
                url="https://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png?lang=fr"
                />
                {contacts.map(contact => (
                    <div key={contact.email}>
                        <Marker position={[contact.lat, contact.lon]} icon={new Icon({iconUrl: contact.is_client ? markerIconClient : markerIconProspect, iconSize: [25, 41], iconAnchor: [12, 41]})}>
                            <Popup offset={[0, -15]}>
                                <p style={{marginBottom: "1vh", textAlign: "center", fontSize: "1.5em"}}>
                                    {contact.raison_sociale}
                                </p>
                                <Button variant="sub" className="mx-auto" style={{border: "teal", borderRadius: "15px", fontSize: "1em", display: "flex", justifyContent: "center"}}>
                                    <a 
                                    href={"https://www.waze.com/fr/live-map/directions?navigate=yes&to=ll." + contact.lat + "%2C" + contact.lon}
                                    style={{textDecoration: "none", color: "black", backgroundColor: "#D0FCB3"}}
                                    >
                                        Allez-y
                                    </a>
                                </Button>
                            </Popup>
                        </Marker>
                    </div>
                ))}
                <SetMap />
            </MapContainer>
        </>
    )

Answer 1

Check if there are same emails in data 'contacts'. Make sure each key for div is unique.

Answer 2

Remember that undefined is also a key-value.

If you have unique emails in your DB, but not required, then there might be several people without email.

I normally go for the _id/id or concider using several fields to build a key. You could also do key={contact.email ?? contact.phone} Or another attribute if email is undefined...