I witnessed today, on the tube, a sort of "passback" occurring. Person goes through exit barrier, then inserts ticket into entrance barrier but doesn't go through. Ticket passed back to second person, who attempts to exit.
It doesn't work as the passback setting on the gates has prevented it.
In the end, the person stuck behind the barriers decided to just push them open, after which two
BTP▸ officers stopped him and directed him to the member of barrier staff.
If you have a season, rover or a travelcard passing through twice in the same direction is entirely possible, and I presume the barriers allow this.
It is allowed, but the barriers won't accept this until after a period of time has expired. There are some exceptions, e.g. at Waterloo, even though most platforms are behind a single gateline, the barriers are "grouped" so you can interchange across the concourse to distant platforms.