I looked up an example sentence of how to use sokkuri and this is what I found.
彼女の言うことはそっくり本心からとは言えない。
She is not quite sincere in what she says.
However, I changed kanojo to anata and said it to my friend and he told me it didn't make sense but he didn't have time to explain why. Thanks for your help!
First, そっくり has two different meanings:
Your example sentence uses そっくり in the second sense, which is relatively less common (and probably is getting less and less common). Your example sentence makes sense as a written Japanese, but sounds awkward in a conversations between young people. I feel そっくり is rarely used in the second sense in conversations except in a few certain idioms such as そっくりそのまま. Perhaps your friend tried to interpret そっくり in the first sense, and thought it made no sense.
Please learn how to use そっくり in the first sense in case you didn't. For the second sense, keep in mind it's a bit literary, and less common expression. You can find some real examples of そっくり here, but these are from novels written more than 50 years ago.
The English expression for this will be "Take what she says with a grain of salt", that is, "She does not exactly/entirely tell the truth (,so take it as such)".
It is possible to say "I will take what you say with a grain of salt" but it would be rare or rather challenging, basically saying in one's face "I do not entirely trust you".
The structure of the sentence is:
[What X says] ≠ [X's true idea/feeling/thought].
It is possible to replace X with 'You', but again to say so in the face of someone would be too direct. It is possible to replace with 'I' but basically saying "I do not entirely tell the truth".
