Steps to reproduce:
1. Conpile cvs gtk+ 
2. Run gedit or gnome-terminal, etc..
3. Crash


Stack trace:
laptop ~% gdb gnome-terminal
GNU gdb 6.4
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This GDB was configured as "i686-pc-linux-gnu"...Using host libthread_db library
"/lib/libthread_db.so.1".

(gdb) r
Starting program: /usr/bin/gnome-terminal 
[Thread debugging using libthread_db enabled]
[New Thread 16384 (LWP 20398)]

(gnome-terminal:20398): GLib-GObject-WARNING **: gsignal.c:1249: signal
"tab_added" already exists in the `GtkNotebook' class ancestry

(gnome-terminal:20398): GLib-GObject-WARNING **: gsignal.c:1249: signal
"tab_removed" already exists in the `GtkNotebook' class ancestry

Program received signal SIGSEGV, Segmentation fault.

+ Trace 66775

Other information:
See also bug 333211

This MUST be considered API breakage. There are several apps that will break not
to mention commercial apps which we have no control over to recompile. GTK+
should check if the callback already exists and not overide, or add it to the
list of callbacks.