Why I need to click twice to destroy a top level window?

hi, guys:

I wrote two files
a. tcl and b.tcl

in my a .tcl:

I wrote:
--------------------------------------------------
exec wish.exe b.tcl
--------------------------------------------------


in my b.tcl:
I wrote:
--------------------------------------------------------
set title "mode"
wm title . $title

frame .main -width 200 -height 300 -bg red
pack .main
tk_optionMenu .main.opt1 say_hello "hello" "hi" "haloha"
set say_hello "hello"
pack .main.opt1 -side top
set bot_frm [frame .main.frm]
pack $bot_frm -side bottom -fill x -expand yes

button $bot_frm.ok -text "Ok" -command "destroy_win"
button $bot_frm.cancel -text "Cancel" -command ""
pack $bot_frm.ok $bot_frm.cancel -side left -fill x -pady 5

#=======================
proc destroy_win {} {
#========================

destroy .

}


----------------------------------------------------------------------------



while when I run my a.tcl, the dialog appears. And I click "Ok", then
anther dialog appears, it's exactly the same dialog with the first one,
then I need to click "Ok" button on the newly generated dialog, and
then both of the two dialog disppear.


Why? Can anybody help me ?
By the way, even I replace 'destroy .' with "wm withdraw ." The same
thing will happen.

.