Light DOM slot forwarding fails to render for duplicated slot names

[nolanlawson]

This is an issue introduced by #4258. It affects light DOM components, and specifically named slots whose slot names are forwarded, and only when those slot names are duplicated in the containing template.

The bug is a bit hard to describe, so it's easier to illustrate with a repro: 5035243

The key component here is header.html:

<template lwc:render-mode="light">
    <div lwc:if={isTablet}>
        <slot name="navigation"></slot>
    </div>
    <div lwc:else>
        <slot name="navigation"></slot>
    </div>
</template>

Note the duplicated slots with the same name (navigation). This is the key thing that triggers the bug when slot forwarding is present. If the condition (in this case isTablet) is toggled, then the content that should be rendered inside of the <slot> is not rendered.

[git2gus]

This issue has been linked to a new work item: W-16441704