If I add multiple slots with the same name it shows all of them.
<div id=t>
<template shadowrootmode=open>
<slot name="s1">s1</slot>
<slot name="s2">s2</slot>
<slot name="s3">s3</slot>
</template>
</div>
<script>
setTimeout(_=> t.innerHTML += `<span slot="s3">three</sp>`, 1000)
setTimeout(_=> t.innerHTML += `<span slot="s1">one</sp>`, 2000)
setTimeout(_=> t.innerHTML += `<span slot="s2">two</sp>`, 3000)
setTimeout(_=> t.innerHTML += `<span slot="s1">ONE!!</sp>`, 4000)
</script>