The plan (and it's the right one) is to have two, and in places maybe three, continuous tracks from end to end.
Even that plan isn't foolproof - sections do get taken out of service for repair - but it's sufficient to reliable run 2WAD service. Then trains in one direction don't have to head in, stop, and wait for trains coming the other way. Each direction has its own track.
Single track lines with sidings do exist for Regional services in other countries, but RER is closer to a 'subway' model than that. The added track is money well spent.
- Paul