Past modals - have (had) gone

Could anybody please explain to me the difference between these two sentences?
"I shouldn't have had done it."
"I shouldn't have done it."

I'm sure about the meaning of the second one, but I don't know if the first is even correct and when to use it, especially in contrast with the second sentence.

CarloSsS said:

Could anybody please explain to me the difference between these two sentences?
"I shouldn't have had done it."
"I shouldn't have done it."

I'm sure about the meaning of the second one, but I don't know if the first is even correct and when to use it, especially in contrast with the second sentence.

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The first is incorrect.

"I should not have done it" is used after you have done something and you want to say that it was a bad idea, or perhaps it was illegal and that (probably) you regret it.

I was angry with my mother and I threw a plate at her. I shouldn't have done it. I could have injured her very badly. It was lucky that I missed her.

I drank two pints of beer last night and then I drove my car. I shouldn't have done it. I know that it's wrong and dangerous to drink and drive.

I drank twelve pints of beer last night and got very drunk. I shouldn't have done it because I had to get up very early this morning to go to work and now I feel dreadful.

Thank you. I knew there was something weird with the first sentence, I've never seen it before. Your rather extensive explanation of the second one was unnecessary as I stated that I know its meaning, but thanks anyway.

CarloSsS said:

Thank you. I knew there was something weird with the first sentence, I've never seen it before. Your rather extensive explanation of the second one was unnecessary as I stated that I know its meaning, but thanks anyway.

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I saw that you said that you understood the first, but other learners will read this thread and won't have known the meaning. Giving a definition and some examples helps more people.