Males set on pupae. Increase the chance that your sperm will do the fertilizing. The problem is greatest where there is internal fertilization. With external fertilization, males are often aware if they are fertilizing eggs. About all they can do to increase chances of fertilization is to increase number of sperm and proximity of release timed to the release of eggs.
Sneakers can dilute male s sperm, but the effect is usually minor.
The evolution of an intromittant organs and sperm recepticals and storage organs is a fascinating topic that we will try to get back to. In most species with internal fertilization, sperm mixing is not complete. May be the last male to mate that gets most of the fertilizations, or it could be the first. Depends on the species. Describe how studies are done to determine Sperm Precedence.