Is John’s baptism invalid?

Further details from the questioner:

In Act 19:1-7 we read that Paul baptizes some Brethren who had already been baptized by John, does this make John’s baptism invalid, and if so, what about Christ’s own Baptism?

The validity of a baptism depends not on who is the baptizer, but on the person being baptized.

Although Paul baptized those disciples (even although previously baptized by John), no mention is made of whether or not Apollos (who had also been baptized by John) was baptized by Pricilla and Aquila.

Act 18:24-28  Now a Jew named Apollos, a native of Alexandria, came to Ephesus. He was an eloquent man, competent in the Scriptures.  (25)  He had been instructed in the way of the Lord. And being fervent in spirit, he spoke and taught accurately the things concerning Jesus, though he knew only the baptism of John.  (26)  He began to speak boldly in the synagogue, but when Priscilla and Aquila heard him, they took him and explained to him the way of God more accurately.  (27)  And when he wished to cross to Achaia, the brothers encouraged him and wrote to the disciples to welcome him. When he arrived, he greatly helped those who through grace had believed,  (28)  for he powerfully refuted the Jews in public, showing by the Scriptures that the Christ was Jesus.

We cannot be certain of the position of Apollos following his baptism by John. However there can be no doubt of the position of Jesus.

Mat 3:13-15  Then Jesus came from Galilee to the Jordan to John, to be baptized by him.  (14)  John would have prevented him, saying, “I need to be baptized by you, and do you come to me?”  (15)  But Jesus answered him, “Let it be so now, for thus it is fitting for us to fulfill all righteousness.” Then he consented.

John was right of course; Jesus is greater than him. If the person performing the baptism needed to be greater than the one being baptized, then he could not have baptised Jesus. Christ’s baptism (by John) was not some sort of legal transaction without which Jesus could not have been saved. His baptism signified the vitally important reality of his own death and resurrection. Importantly Jesus shared our mortality. Our death and resurrection (both actual and baptism as a figure of this) is dependent on our faith in Christ’s death and resurrection.

Rom 6:3-5  Do you not know that all of us who have been baptized into Christ Jesus were baptized into his death?  (4)  We were buried therefore with him by baptism into death, in order that, just as Christ was raised from the dead by the glory of the Father, we too might walk in newness of life.  (5)  For if we have been united with him in a death like his, we shall certainly be united with him in a resurrection like his.

To question the validity of Jesus’s baptism just does not make any sense. However the fact that John baptized Jesus suggests the validity of John’s baptism for other people he baptized, provided their faith in Christ was adequate.