Although it is commonly used by Portuguese speakers, twenty years ago the expression was considered a vicious pleonasm. The correct expression is twenty years ago.
Why is twenty years ago an example of vicious pleonasm? Because there is a repetition of ideas in it, given that the verb to be, when conjugated as verb impersonal, indicates elapsed time, thus making unnecessary the use of the adverb ago to indicate the past.
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Examples:
twenty years ago we got married.
Five years ago We traveled to Italy on our honeymoon.
I'm waiting for you thirty minutes ago!
Ten years ago We met in law school.
Although it is considered as a vice of language, twenty years ago the expression was enshrined in the speech and even in writing, and can be easily found in literary works and letters of music. Some less orthodox grammarians already validate its use, understanding that the use of adverbback reinforces the idea of past tense and facilitates the understanding of the message.
Good studies!