prevention of the same user to run the script twice [multithreading?]

[eluser]majidmx[/eluser]
The tracking system is CI based application.
We have a script for tracking the sales which is being called by an image from the last page of the shopping website.

Shopping website - Last page

Code:

<img src='http://example.com/sales_tracking/index.php/sale/trackSale/' />

But it could happen for a shoping owner to have such a thing :

Code:

<img src='http://example.com/sales_tracking/index.php/sale/trackSale/' />
.
.
.
&lt;iframe src='http://example.com/sales_tracking/index.php/sale/trackSale/'&gt;&lt;/iframe>

and in this case our script will be called twice at the same time.
THe time difference is so small that can not be detected, or maybe that;s because of the threading issue I mentioned earlier...
I want to get some idea how to prevent the second call from within the same page.