Currently, the client will automatically "steal" an existing session, even if there is already another client connected to it. There is no way to change this behaviour, nor for the user to know in advance whether they have an existing session or not (at least not when max_sessions_per_user=1).
This is not always desirable behaviour. For example, if the existing session contains a running job which requires resources from the already connected client (serial devices, local drives, etc).
One idea would be to detect whether an existing session already has a client connected to it, and if so, present a confirmation dialogue to the user before taking it over. Otherwise, just take it over. This might even be considered sensible enough default behaviour to preclude having a separate option for it.
Bug #879 is also somewhat related.
This bug is a bit vague without more specific scenario where this is a practical problem. Perhaps duplicate of bug 6230?