what this is trying to do is say if there is no source map or we have already processed the source map, continue to the next file.  As an elif you will always catch the `there is no source map` case, but can't catch the `there is a source map but its already been processed` case because there is a source map which will result in the first if being true. 

So the resulting change in behavior is that duplicate source maps will no longer bee processed twice