As late as 1952 this is what the WT said:
*** w52 3/1 p. 159 Questions From Readers ***
? If one has already been baptized, does he need to repeat the baptism after he gets a knowledge of the truth?—R. G., Canada.
Whether a person is to be baptized again or not is determined by his understanding of baptism when he first underwent it. Did he understand the meaning of the symbol of water immersion? Did he fully appreciate that it meant a complete dedication of his life to the Lord, to serve the Lord, to do His will? Had he made such a dedication in his mind and heart and before the Lord prior to the immersion in water, which is a public symbolizing of the previously made dedication? If so, and if the baptism was a complete submersion in water, then there is no necessity for the person to perform the symbol again. The one who does the immersing, the place of immersion, and who are present as spectators, are not the determining factors. It is the proper understanding and appreciation of baptism on the part of the one being immersed that counts. If the person did not have this understanding and appreciation, if he merely viewed baptism as a religious ceremony affiliating him with a certain church, not realizing the meaning or importance of the step and what would be thereafter required of him from a Scriptural standpoint, then when such a person comes to a knowledge of the truth and wishes to dedicate his life to the God of truth as one of His witnesses the person should symbolize this dedication that he has now made with understanding.
But then the 1956 WT backtracked:
*** w56 7/1 p. 406 Why One Must Be Baptized ***
BAPTIZED A SECOND TIME
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Often the question is asked whether one baptized previously in a ceremony performed by some other religious group should again be baptized when coming to an accurate knowledge of the truth and making a dedication to Jehovah. Because of what has been already said, now there is compelling reason for also saying, Yes, one must be baptized again. Obviously, by any of such religious systems one was never in reality baptized "in the name of the Father and of the Son and of the holy spirit," because had he been so baptized he would have appreciated the authority and office of such true Higher Powers. And if previously dedicated to Jehovah, the individual would have separated himself from such God-dishonoring Babylonish systems even before letting them baptize him. So the act of being baptized is not the important thing, but, rather, that which the act symbolizes is the element of importance.