If YOU forgive the sins of any, they are forgiven them

Who were the 'you'?

ESV, Jn 20:

19 On the evening of that day, the first day of the week,

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resurrection Sunday

the doors being locked where the disciples were for fear of the Jews,

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Thomas was not there.

Jesus came and stood among them and said to them, “Peace be with you.” 20 When he had said this, he showed them his hands and his side. Then the disciples were glad when they saw the Lord. 21Jesus said to them again, “Peace be with you. As the Father has sent me, even so I am sending you.” 22 And when he had said this, he breathed on them

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He gave them a special function of the Holy Spirit.

and said to them, “Receive the Holy Spirit. 23 If you forgive the sins of any, they are forgiven them; if you withhold forgiveness from any, it is withheld.”

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These particular disciples [D1] were given a special revelation.

24 Now Thomas, one of the twelve, called the Twin, was not with them when Jesus came.

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Does this mean that every Christian has the power to forgive and retain sins independently from God?

No, Thomas didn't have this. Even for D1, they didn't have this power. They had a special revelation concerning particular individuals' sins God would and would not forgive.

New American Standard Bible, John 20:

23 If you forgive the sins of any, their sins have been forgiven them; if you retain the sins of any, they have been retained.”

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they have been forgiven;
ἀφέωνται (apheōntai)
Verb - Perfect Indicative Middle or Passive - 3rd Person Plural

Who were the 'you'?

They were the few disciples [D1] that Jesus breathed on. Jesus gave them the ability on particular occasions to declare whether a sin was forgiven or not by God. Today, we do not have the power to forgive or retain sins at will. However, under the inspiration of the Holy Spirit, there are occasions when we can see another's spiritual condition and declare it.