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If I want to say that I agree with XYZ's opinion, how do I say that?

1) I second XYZ
2) I second XYZ's opinion
3) I second XYZ's thought

Which one is the correct way?

/Sameer
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Hi Sameer

Probably the most common form of this expression is the one you wrote in the title of the thread:
"I second that."

This expression is frequently used to show support for a proposal or suggestion that someone has just made, but might also be used to agree with an opinion. The word "that" in the expression basically means "that which has just been said" or "that idea".
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I am in 100% agreement with Yankee. "I second that" is what you would say, if XYZ has just spoken.

If YXZ speaks, then ABC, then DEF, and now it's your turn, you can say any number of things: I agree with YXZ. I hold the same opinion as XYZ. XYZ's suggestion makes the most sense to me. XYZ's statement summed up my thoughts perfectly.
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Comments  
'Second' in this context is usually used to show support some kind of idea or proposal, rather than a person. I think (2) would be the best choice.

Let's see if someone seconds my proposal. Emotion: smile
 Yankee's reply was promoted to an answer.
I Second XYZ is the most appropriate way to Second some one.
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I think it is "I second XYZ's opinion"
 BarbaraPA's reply was promoted to an answer.