Intransitive verb that becomes passive?

Question

What are the nuances when an intransitive verbs becomes a passive verb. Is not it just duplication:

Example

窓が開いた

And

窓が開かれた

Isnt 開かれる and 開く both mean "to be opened".

Answer 1

窓が開いた
And
窓が開かれた

Do you mean 窓が[開]{ひら}いた and 窓が[開]{ひら}かれた ? If so, the [開]{ひら}かれた is NOT the passive of intransitive [開]{ひら}いた but the passive of transitive [開]{ひら}いた.

Active: 窓を[開]{ひら}く "(someone) opens the window" ← [開]{ひら}く is transitive
⇨ Passive: 窓が[開]{ひら}かれる "the window is opened (by someone)"

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If you mean 窓が[開]{あ}いた and 窓が[開]{あ}かれた, then your second sentence is grammatically incorrect.

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As you may know, [開]{あ}く is intransitive, [開]{ひら}く can be transitive and intransitive, and [開]{あ}ける is transitive. To sum up:

◎「窓が[開]{あ}く」「窓が[開]{ひら}く」-- intransitive "the window opens"
◎「窓を[開]{ひら}く」「窓を[開]{あ}ける」-- transitive "to open the window"
◎「窓が[開]{ひら}かれる」「窓が[開]{あ}けられる」-- passive of 窓を[開]{ひら}く and 窓を[開]{あ}ける. "the window is opened"
✕「窓が[開]{あ}かれる」-- incorrect

Related:

• Distinguish between homonyms & potential form
• ひらく / とじる vs. あける / しめる