Not sure if we care. Do we want to allow // i18n ( ph = "second" )?

I suppose we do. Probably would be less confusing for people. I'll update and add a test.

This is a nitpick, so feel free to ignore it. It's better avoid reassigning parameters. It adds unnecessary mutability. It's harder to follow the flow of the computation. In this case it doesn't matter that much, but it may matter in other cases.

export function dedupePhName(usedNames: Map<string, number>, name: string): string {
  let duplicateNameCount = usedNames.get(name);
  if (isPresent(duplicateNameCount)) {
    usedNames.set(name, duplicateNameCount + 1);
    return `${name}_${duplicateNameCount}`;
  } else {
    usedNames.set(name, 1);
    return name;
  }
}

Ha! That's how I had it originally. I'll change it.