When you translate the content, you must click on the "+" sign. If you duplicate the post / page, when you edit the original content, it will be synchronized with the duplicate posts and overwrite translated content.
To avoid this behavior, you must open the translated post editor and click the "Translate Independently" button.
About String Translation, in WPML > Theme and localization plugins, make sure that the 'Translate by WPML' is selected. This is necessary so that the string Translation works as expected.
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