bug#51941: Detect duplication of ERT tests

[Ihor Radchenko]

Mattias Engdegård <mattiase@acm.org> writes:

> Best practice is to run each ERT test file in a separate Emacs process. This 
> ensures isolation between the tests, which can be quite important, but also 
> permits them to be run in parallel. Easiest is to do this from a Makefile but 
> you could of course do it from a controlling Emacs process if you prefer that.

Makes sense, though it is not trivial to change the existing code.

> Now if you for some reason want to stick with running them all in a single 
> process, which I would advice against, then at least avoid the load-file if 
> the file has already been loaded. Maybe use `require` with a file argument 
> instead of load-file.

Using require indeed solved the issue.

Best,
Ihor