It seems to be okay without "ever", but is "never ever" have more stronger meaning than just "never"?
Yes, "ever" in this case is used to make "never" sound stronger. However, it is common in English to put the word "ever" after "never" to give it an even stronger meaning. Sometimes in English, people use "ever" multiple times for emphasis. Ex: I will never ever leave you. Ex: You are never ever ever going to beat me.
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