It is not possible to identify whether a given script has been added by JsFileLink control ( CssFileLink adds import or link attributes) without looping through header controls collection. JsFileLink and CssFileLink controls do not make such check before adding a given tag inside Render method( for embedded scripts ) or CreatechildControls( for custom scripts). Using IsClientScriptBlockRegistered would be useful if it is done inside one of the methods above with conjunction of RegisterClientScriptBlock
You could override Render of InternalPage, but here you should loop through the controls collection again.
Basically you could create your own custom control that acts in this way
var scriptText =
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