Why not use cookies to check if the user has pressed F5 or refreshed the 
page?  There is a number of ways to do this, all should work effectively 
depending on your how many people hit the page etc.
  a) Store the message in a cookie, and run PHP code to check before executing 
the mail() function.  If the message matches after a refresh than don't execute 
the mail() function.  You could display the message again in the box or setup a 
error reporting message saying you cannot refresh the page or send the same 
message twice.
  b) Everytime the page is loaded (whatever.php) from a weblink or something 
have a hidden field like id=rand() ... Use a random generator to randomly 
uissue lets say a 12 char legnth.  So if the user refreshes the hidden field 
would be the same hence no mail() execution.  However if the page is loaded 
from a weblink the php issues a new code.
  c) I did have a c.. but forgot.  Look up the following on google "checksum or 
  Anyways.  There is more ways but it's time for me to hit the sack(). :P

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