My pony does this alot. He was taught that it was the only way to jump by previous homes, and so that's all he tries to do. Usually he'll go up to a single jump just fine, but when we're working on lines he just explodes to the second fence. I've worked alot on halting him in between the lines, and we basically do NOT go over that second jump, even if I have to circle him out before we halt. then, he can walk over the jump if it's low enough, ie a crossrail, or trot it if it's higher. Usually after repeating this exercise a few times, he goes through the lines like a short stirrup pony. And he's never stopped after a jump.