Standard kanji spelling; the repetition mark 々 is preferred in modern writing.
なみなみUsage46%
並み並みUsage1%
Explicit kana-kanji mixed spelling; rarely used.
並並Usage1%
Less common variant without the repetition mark; still encountered.
Etymology
Reduplication of 並 (なみ, 'average, ordinary'), emphasizing the quality of being ordinary. The exact historical derivation is uncertain, but the form is well-established in classical and modern Japanese.