or insuficient slope on the wate pipe..
That isn't the job of an overflow.
you can get a bottle trap that is meant for syphoning problems, Marley about
5 quid in B&Q. I doubt it will help your problem thoughmaybe a fundemental design problem with the basin. Often the fancy looking items are a bit impractical like one toilet I fitted not too long ago, looks great but blocks very easily..