email tracking image duplicate requests

I am embedding tracking images within emails that are being sent from a custom-built opt-in CRM system. The image src is an encoded .gif, such as src="12_34_675.gif". The image is served by an ASP.NET httphandler that decodes the src encoding and serves a transparent image.

Everything works fine, but some email clients request the image multiple times, creating duplicate entries. Some clients make three calls all within one second, and some seem to make tens of calls over a day or so. Mostly email clients make single calls, but these few duplicates are very perplexing. I know I can code around them, but I'd really like to understand what's going on.

I've checked the IIS log files, which 开发者_开发技巧show that the duplicate requests are coming from the client machines. I can't think what might be causing these duplicate http requests.

Help!

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I don't think this is something you can control. What if they have an old version of outlook open (older version used to open images embedded in message by default, nice) with the preview pane and pass through your email a few time. I am sure you are not using the tracking data directly from urchin or whatever you are using. Is there a reason the duplicate log entries for the image are a problem?

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I actually have this same problem now and I'm not sure why.

in my code I redirect using mod_rewrite to a tracking script and this script basically parses the get params for which campaign and which contact list etc, then I write some data to the database.

at the end of the script, I output the image using code like

header("Content-Type: image/jpeg");
readfile($filename);

$filename is the correct file, I checked it, var_dump'd it and the script does output the correct file.

in my access logs, I get TWO hits on the script, I get TWO duplicate records in the database and all my stats are double what they should be.