addResults should only callback once

[matthewp]

This will cause Node to crash because async throws an error as shown
here. Fix
is to use isaac's once module to make sure we only call next one time.

[indexzero]

👍 I'd prefer not to take an extra dependency but you are correct that the Logger should be more defensive about working with transports in this scenario. We can't trust that every transport only invoke it's own callback once.

[matthewp]

I think this is a reasonable dependency to add though. It's 20 LOC, has 1 dependency of it's own (also very small) and is written by isaacs... but feel free to rewrite that logic yourself if you prefer.

[indexzero]

This is paving over a larger problem of why these would get called multiple times in the first place. I would prefer to make this the sole exception and not a common occurrence. If we have to implement once-like functionality enough to need a module, we have other much bigger problems.