Fix EZP-23071: duplicate jquery inclusions break jquery plugins

https://jira.ez.no/browse/EZP-23071

When f.e. a page using ezie edit attribute is rendered, there are multiple jquery inclusions, one of them at the bottom of the javascript list.
This causes the jquery plugin (that extends `$.fn` with new function(s)) to be ignored.

Fix by making sure there are no duplicate scripts in ezjscore's javascript tag output.