The Tor Browser / file revision

 // After clearing one breakpoint, another breakpoint at the same instruction still works.

 var g = newGlobal();

 var dbg = Debugger(g);

 var script = null;

 var handlers = [];

 dbg.onDebuggerStatement = function (frame) {

     function handler(i) {

         return {hit: function (frame) { g.log += '' + i; }};

     }

     var f = frame.eval("f").return;

     var s = f.script;

     if (script === null)

         script = s;

     else

         assertEq(s, script);

     var offs = s.getLineOffsets(g.line0 + 3);

     for (var i = 0; i < 3; i++) {

         handlers[i] = handler(i);

         for (var j = 0; j < offs.length; j++)

             s.setBreakpoint(offs[j], handlers[i]);

     }

 };

 g.eval("var line0 = Error().lineNumber;\n" +

        "function f() {\n" +     // line0 + 1

        "    log += 'x';\n" +    // line0 + 2

        "    log += 'y';\n" +    // line0 + 3

        "}\n" +

        "debugger;\n");

 assertEq(handlers.length, 3);

 g.log = '';

 g.f();

 assertEq(g.log, 'x012y');

 script.clearBreakpoint(handlers[0]);

 g.log = '';

 g.f();

 assertEq(g.log, 'x12y');