Postdissector executes more than one time per packet?

I'm trying to use Postdissector, but I found that postdissector may be triggered several times per packet captured. I tried the following code and it proved the fact. I'm a newbie to lua in wireshark and i'm not sure if there's something else needs to be done. Glad if anyone can help~

do
test_proto = Proto ("test", "Test Protocol")
function test_proto.dissector (buffer, pinfo, tree)
tw:append(tostring(pinfo.abs_ts))
tw:append("\n")
end
register_postdissector(test_proto)
tw = TextWindow.new("Debug")
end

something in the debug window: 1321624108.557 1321624108.557(this is the 2nd occurence) 1321624108.5764 1321624108.5764(and this)