Does playing a field spell destroy an existing field spell? - Answers

Yes, there can only be one active at any one time. If you already have a Field Spell active, you can replace it with another, the first is destroyed when you activate the second. If the opponent has one, then it is destroyed when you Field Spell card resolves. Field Spell cards can be set while another is active. Since the rule only affects active Field Spells, set cards are not destroyed by your opponent setting or activating a Field Spell card.