Receive the Holy Spirit

Why does Jesus breathe on them and tell them to "Receive the Holy Spirit" (John 20:22) now?
He had just given them their marching orders - “As the Father has sent Me, I also send you” (John 20:21) - and the Holy Spirit will empower and guide them to accomplish what they are being sent out to accomplish.

Don't they receive the Holy Spirit again a few weeks later?
They and other disciples not only receive but are "filled" (Acts 2:4) with the Holy Spirit fifty days later at the Pentecost when God the Father and Jesus "pour out" (Acts 2:17) the Holy Spirit on them after Jesus ascends to heaven. This is a period of transition after Jesus' resurrection and before His ascension, after which the era of the Holy Spirit's ministry on earth will begin. The apostles receiving the Holy Spirit as Jesus breathed on them will take them through until they are filled with Him fifty days later.

Why does Jesus "breathe" (John 20:22) on them all of a sudden?
God breathing on those who belong to Him to spiritually awaken and empower them is nothing new: "And the Lord God formed man of the dust of the ground, and breathed into his nostrils the breath of life; and man became a living being" (Genesis 2:7).

Who can forgive sins?
Only God: "Who can forgive sins but God alone?” (Mark 2:7)

Then why does Jesus say to His disciples, “Whose soever sins you forgive, they were forgiven them; whose soever sins you retain, they were retained” (John 20:23)?
Jesus has been clear throughout His ministry on whose sins are forgiven and whose sins are not forgiven. As He now sends them out, He is commissioning them and expressing His trust and confidence that they will carry forth His message on the forgiveness of sins that accords with His instructions.

Why didn't He just tell them that He forgives sins as per His instructions in the Bible?
Those who will record His instructions as the New Testament of the Bible were His apostles, most of whom were in the room when He spoke the words above.

What were His instructions on whose sins are and aren't forgiven?
The sins of those who repent from them and believe that Jesus died on the cross to pay the death penalty due for their sins are forgiven, while the sins of everyone else are retained, to be paid by the sinner himself or herself in hell (see John 3:16).

What do we call people who claim that Jesus gave His apostles the right to make up on their own which sins to forgive and to not forgive, and that that right has been passed down to them and to no one else?
A cult.