debugger "bug"?

I have noticed something odd with the debugger that might be a SF bug, or maybe just a quirk of the debugger. 

If you set a breakpoint in a program, say on line 9, and then later add a line of code above (before) the line with the breakpoint, and then rebuild and run under the debugger, two breakpoints (as red dots) will appear in the source code pane , the original one on line 10 (the old line 9), and another one on line 9 (what used to be line 8).  Unfortunately, the only "active" breakpoint is the one on the new line 9 and not the intended place (line 10).

I checked this under Windows and Linux, same behavior.