Python e-mail CGI script sends duplicate e-mail

I've been experimenting with a Python CGI script to send an e-mail (hosted with a comercial web host - 123reg), and the problem is whenever I run the script from my web browser, it sends two identical e-mails.

The code to send the mail is definitely only being executed once, there are no loops which could cause it to happen twice, I am definitely not clicking the button twice. No exceptions are thrown and the "success" page is sent to the browser as normal.

The strangest thing is that when I comment out the code to print the result page (which is very simple and has no side effects, just 3 print statements in a row) and replace it with a dummy print statement (print "Content-type: text/plain\n\ntest"), it works properly and only sends one e-mail.

I have tried googling the problem to no avail.

I am at my wit's end because this problem doesn't make any sense to me. I'm pretty sure it must be my script since inexplicably it works when you comment out those print statements.

I'd appreciate any help, thanks.

EDIT:

Here's the code which, when commented out, fixes the problem:

  print "Content-type: text/html"
  print
  print page

EDIT:

The code to send the e-mail:

#send_email function: sends message from from_addr, assumes valid input
def send_email(from_addr, message):
  #form the email headers/text:
  email =  "From: " + from_addr + "\n"
  email += "To: " + TO[0] + "\n"
  email += "Subject: " + SUBJECT + "\n"
  email += "\n"
  email += message

  #return true for success, false for failure:
  try:
      server = smtplib.SMTP(SERVER)
      server.sendmail(from_addr, TO, email)
      server.quit()
      return True;
  except smtplib.SMTPException:
      return False;
#end of send_email function

I'd post the code to format the page variable, but all it does is read from a file, format a string and return the string. Nothing unusual going on.

EDIT

OK, I've commented out the file IO code in the create_page function and it solves the issue, but I don't understand why, and I don't know how to modify it so that it'll work properly.

The create_page function, and therefore the file IO, was still being executed when I found that commenting out the print statements solved the problem.

This is the file IO 开发者_JAVA技巧code from before I commented it out (it's at the very start of the create_page function and the rest of the function simply modifies the page string, then returns it):

  #read the template from the file:
  frame_f = open(FRAME)
  page = frame_f.read()
  frame_f.close()

EDIT:

I have just replaced the file IO by copying and pasting the file text directly into a string in my source file, so there is no longer any file IO. This still hasn't fixed the problem. At this point my only theory is that computers hate me...

EDIT:

I'll have to post this here since stackoverflow won't let me answer my own question since I'm a newbie here...

EDIT:

OK, I posted it as an actual answer now.

PROBLEM SOLVED!

It turns out that it was the browser's fault all along. The reason I didn't notice this sooner was because I tested it in both Firefox and Chrome ages ago to rule the browser out, however it turns out that both Chrome and Firefox share this same bug.

I realised what was happening when the server logs finally updated, I realised that often GET requests were immediately (1 second later) followed by another GET request. I did some googling and found this:

What causes Firefox to make a GET request after submitting a form via the POST method?

It turns out that if you have an img tag with an empty src attribute e.g.

<img src=""/>

(I had some javascript which modified that tag), Firefox will send a duplicate GET request in place of a request for the image. It also turns out that Chrome has the same problem. This also explains why the problem was only happening when I was trying to include my html template.

It would help if you posted more code, but does the "page" variable contain code that would execute the email server a second time, or cause a page refresh that would trigger the email a second time.

The same thing will happen if you have a Javascript call with an empty src or "#" as src:

<script type="text/javascript" src="#"></script>

Perhaps also with an empty href for a css link. I haven't experienced that, but I'd expect the same behavior.