SRD Talk:Duration (Spell Descriptor)
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This page has an error. It says "You can’t cast a spell while concentrating on another one." You can cast another spell. If you cast another spell that DOES NOT require concentration, then you can maintain concentration on the original spell.
However, if you cast another second spell that requires concentration, then you must break concentration on the original spell to begin concentrating on the new spell.
Example: I cast Haste on the fighter, and concentrate on it. Next turn I cast Magic Missile on the goblin. The fighter still has haste because magic missile doesn't require concentration. Then, on my next turn I cast levitate on myself. In order to concentrate on levitate, I have to break concentration on haste, so the fighter loses haste when I cast levitate.