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    <p>There are two reasons why this can happen. The first is that the content provider can set the <code>expires</code> heading so that the browser will not issue a second request, and the second is how browsers actually handle <code>GET</code> requets in HTTP, such as the get request to include the script.</p> <h2>1. Setting Expires Header of Content</h2> <p>The content provider can set the <code>Expires</code> header so that the script is cached by the browser the first time, hence the lack of a second request. This is a good standard web practice for speeding up web pages, and the expires header is set by the host server of a script. Yahoo Developer has a good article on this at <a href="http://developer.yahoo.com/performance/rules.html" rel="nofollow">Add an Expires or a Cache-Control Header</a> which recommends adding the Expires header to scripts and stylesheets in addition to images.</p> <h2>2. Script Get Method is Idempotent</h2> <p><code>GET</code> requests, such as the get request to include the javascript on a webpage are safe methods according to the HTTP specification. Safe methods are also idempotent, in that multiple requests yield the same result as a single request and that the method is only used to retrieve data. Many browsers take advantage of this property of the HTTP specification, and won't sent multiple requests for idempotent methods. George Cummins explained this well, and an article on this is available from the <a href="http://developer.mozilla.org/en-US/docs/HTTP" rel="nofollow">Mozilla Developer Network</a>.</p>
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    1. POWhy duplicate script tags do not produce duplicate requests?
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    1. POWhy duplicate script tags do not produce duplicate requests?
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    1. COI've tried It even happens with the following headers Cache-Control:no-store, no-cache, must-revalidate, max-age=0 Expires:Mon, 04 Dec 1999 21:29:02 GMT Pragma:no-cache I will add a test snippet if neccessary
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    2. COGet requests, such as the get request to include the javascript on a webpage are safe methods according to the HTTP specification. Safe methods are also idempotent, in that multiple requests yield the same result as a single request and that the method is only used to retrieve data. Many browsers take advantage of this property of the HTTP specification, and won't sent multiple requests for idempotent methods. George Cummins explained this well, and an article on this from the Mozilla Developer Network is available here: https://developer.mozilla.org/en-US/docs/HTTP
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    3. COI have edited the response above to include the above comment for clarification.
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