Two base running topics that pop up every once in a while, but seem to confuse everyone anyway are when two runners end up on the same base and when one runner passes another.
When two runners end up on the same base it is the lead runner that is entitled to the base; therefore the trailing runner upon being tagged shall be called out. The exception to this rule comes in the event of a runner being forced by the batter. If a player starts out on first base and remains there for whatever reason while the ball is put into play, the defense has the option to touch second base with the ball (getting the common putout) or tagging the runner.
One aspect of this rule makes it one of the oddest to think about—the out is not automatic. In the hypothetical event that two runners end up on the same base and the defense tags neither of them, then neither one is out. Pushing this situation to its extreme would allow for the bases to be filled with every runner in the lineup and allow a batter to hit a 9-run home run (Double Grand Slam? Ultra Slam?). Any defensive team that wants to win would never allow this to happen—two runners ending up on one base is essentially a free out for the defense so they would never turn it down. Also the umpire would have the option at any time to make some type of judgment to return the game to normality.
Passing Another Runner
If one runner passes another on the base path, the runner who did the passing (generally the faster runner) is the one that gets called out. It’s to his advantage to try to run back to the previous base, because as we see above if he ends up on the same base as the runner in front of him he will be out anyway.