SERVER-119774 changed the behavior of loading extensions through resmoke to append the list of extension names to mongod/mongos configuration rather than overwriting the existing list. This was to accommodate situations in which we specify extension directly in the YAML suite configuration (e.g. `mongot-extension`) and through resmoke.

However, there are some previously unrecognized situations in which the call to `_append_to_load_extensions()` was called twice upon spinning up the testing environment, resulting in extensions being in the `loadExtensions` list twice (this is an error, see linked BF).

We should prevent this double append while maintaining the functionality of not overwriting the `loadExtensions` list entirely.