This has to do with how your browser handles resource requests. Flash has similar issues and there are a couple workarounds.
Here's an article that details the issue and possible solutions.
I would suggest doing something like this:
Say you have this for your xap in your html:
<param name="source" value="ClientBin/myApp.xap"/>
I would version it so whenever you do a push you change the version number. Example:
<param name="source" value="ClientBin/myApp.xap?ver=1"/>
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