About [[MediaWiki:Cx-sourceselector-dialog-error-page-and-title-exist/nb]]
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This is too unspecific, the translator must be told which page this is. It could be that the page was already translated. Probably the whold message should be rephrased and an additional arg added for the target page title.
As you can see in MediaWiki:Cx-sourceselector-dialog-error-page-and-title-exist/qqq, all details are available:
MediaWiki:Cx-sourceselector-dialog-error-page-and-title-exist/qqq