Actually, there are three possibilities:

• Two normal ==Heading==s conflict – handled by the parser.
• Two <span>s generated by us conflict – this is what you scoped this task to.
• A normal ==Heading== conflicts with a <span> generated by us – what about this?

Even worse, it may happen that an ID generated by the parser to resolve a conflict ends up conflicting with our ID, like in the following case:

<translate>
== Foo 2 == <!--T:1-->

== Foo == <!--T:2-->
</translate>

<span id="Foo_2"></span>
<div class="mw-translate-fuzzy">
== Foo ==
</div>

<div class="mw-content-ltr" lang="en" dir="en">
== Foo ==
</div>

Since ==Foo== appears twice in the translation, the parser-generated ID of the second one will be Foo_2, just like the ID we generate for the first one. This even breaks the TOC, as it’ll contain a link to #Foo_2 for the second heading, but the browser will (likely) jump to the first anchor it finds with the given ID.

(I tried to create a somewhat realistic example. The story behind it: T:1 used to be ==Foo==, but then the heading was changed to ==Foo 2==, fuzzying the translation. Later a new heading (T:2) was added with the text ==Foo==, but since many languages have already updated the translation of T:1 to match ==Foo 2==, the translation admin decided to add a new translation unit for ==Foo==, and not for ==Foo 2==. The translation we’re looking at, however, hasn’t been updated in the last six years, so it still contains ==Foo== as the translation of ==Foo 2==.)