Comic companies often publish "crossover" story arcs. These stories include one or more issues of multiple titles. By subscribing to a crossover in its entirety, you can be sure to get one copy of each issue that is relevant to the story without adding all of the titles to your subscription.
Click "Crossovers" in the left menu for a list of available crossovers. You can click "View Issues" for a list of all the upcoming issues currently assigned to that crossover. Click "Add" to add the crossover to your subscription.
Subscribing to a crossover will never cause you to receive "extra" copies of an issue you are already getting through another kind of subscription.