So even though I have the same objects, array.includes() does not work in Vue unfortunately. My guess is that behind the scenes Vue alters the object, thus includes() sees it as a different object.
export default {
    props: ['sellitem'],
    data(){
        return {
            sells: [ this.sellitem ]
        };
    },
    watch: {
        sellitem(value){
            if( ! this.sells.includes(value)) this.sells.push(value);
        }
    },
}
Periodically the component gets new Prop values so I add them to the array sells. But of course when something is already in the array it should not be pushed inside.
