Overlapping Durations

When a creature is subject to identical effects that end at different times, it ignores all but the effect that has the most time remaining. For instance, if Dendric the fighter is slowed by a monster until the end of the encounter and is then slowed by another monster until the end of his next turn, he ignores the second effect, since it has less time remaining than the first.
    Effects that a save can end (labeled “save ends”) work differently, since it’s not possible to know when they’re going to end. Therefore, effects that a save can end are tracked separately from those that end at specific times.

Published in Player's Handbook, page(s) 278, Rules Compendium, page(s) 227.