TODO: Verify substate exclusivity in templates

I want to verify that the glimmer rewriter enforces that the same state cannot be used on the same element more than once.

Start by locating the Glimmer rewriter and its template handling. Determine how repeated state usage on one element is currently processed, then add verification that the same state cannot be used more than once on that element; the work is done when this behavior is covered and the relevant tests pass.

Written by the indexing model from the issue text.