Posted: 15/04/2014 at 16:28
This is not new - I had someone use the same number as me when I ran in 2003. As someone has stated above, the marshals either stop them before the finish or they get moved on from the finish area so don't get the medals, T-shirts (or get an official time). Yes, they'll get the water, gels & whatever else but it would cause chaos at the water stations if they had to check the validity of each runner.
Did Paris last week and there were some spectators (I saw 2 who did this) who joined in at various stages to accompany their friends. Presumably they pulled away before the finish but I didn't notice. We also had a cyclist join us for some of the route but I think he got moved on.
Is it a problem? A minor one. Marshals can't be everywhere along the route so you can't feasibly stop someone if they want to do a bandit run.