It's kind of interesting what happens if two interfaces have identical signatures, say "public void someMethod( String arg )". When you implement it once you've made both interfaces happy. But you can't tell which interface the caller had in mind when they made the call. Unless the two interfaces promise very similar behavior you run the risk of confusing the caller.
A good question is never answered. It is not a bolt to be tightened into place but a seed to be planted and to bear more seed toward the hope of greening the landscape of the idea. John Ciardi