Four friends walk into a bar

Four friends meet every Friday evening at the bar. They have a ritual; everyone brings up one interesting and intellectually curious anecdote/fact/story from a variety of subjects- arts, history, sciences, math .. as they say, any topic under the sun or snow.

Develop a rigorous sytem or argument to determine if and when there might be a collision of topics - at least two friends bring come up with the same story. (Assume no changing of story at the last minute). Also assume that they have a fair and reasonable definition of what constitutes a collision and no one debates that.