While this answer is more /r/ELINT, I'll answer from a biblical doctrine angle.

Cleanliness is the major point of the Old Testament. God is pure. And to be a member of God's chosen people in the Old Testament, purity was to be expected. This is why there were odd rules about not eating certain kinds of meat or wearing certain garments. It was the job of the High Priests to mediate for the people by approaching God in the temple. To do this, they were instructed to be clean in every way: their clothes, their bodies, what they ate, their hearts etc. including ritual washing as they prepared to enter "the holy of holies" where the High Priest met with God once a year to atone for the sins of the nation of Israel.

"Baptism" comes from the Greek (baptizo: to plunge, dip or immerse). With most of the OT written in Hebrew and much of the NT written in Greek, Matthew (1st book in the NT) is the first place in the bible we encounter the word Baptism or Baptist in the character of John.

In the New Testament, Baptism is constantly associated with repentance or confession. Very rarely are the two terms far apart especially with regards to John the Baptist. He also said "I have baptized you with water, but he will baptize you with the Holy Spirit." Early in the NT (during Jesus' life), baptism is introduced as taking place in a river and as a reaction to confession or repentance. Later after Jesus's ascension, Acts 2 describes the first Christians getting baptized in the Holy Spirit and no water is present. Following this event, baptism is seen often among early Christians immediately following a conversion and often with water. When someone is converted to Christianity (Simon the Magician, and the Ethiopian eunuch [Acts 8], Saul of Tarsus [Acts 9], Paul and Silas's jailer [Acts 16] etc), they are immediately prompted/encouraged to be baptized.

Nowhere in the New Testament is baptism paralleled to Old Testament purification. Old Testament cleanliness was preventative, New Testament Baptism is reactionary. Therefore, while many historical religious ceremonies introduce the step of "ritual washing", Baptism as introduced in the New Testament (first through John the Baptist) is unlike most religious washings. Baptism is in response to the singular event of conversion.

I don't have enough time to make a long answer right now (boutta sneak off to Gran's from some of that Thanksloving), but short answer is no. Baptismal fonts were discovered at Qumran (think Dead Sea Scrolls), which existed ca. 100 BC, but scholars think it was more of an ablution ceremony than a covenant with Christ ordinance.

It's unclear that baptism was only administered once. Reread the parts of the New Testament that involve John the Baptist (Wikipedia has a list. You certainly can read them as something that happened once, but you can also read them as something where if you sin again, you have to repent again (and get baptized again). That's probably more in line with the evidence we have from Qumran--that ritual immersion was an important and regular practice for at least some Jews of this period.

But neither in the New Testamanet and Josephus (our two accounts closest chronologically to the historical John) is there a once and done implication. Indeed, the later explanations for why Baptism is a single transformative event--either the erasure of original sin, accepting Jesus as one's savior, etc--aren't implied in the texts. Look instead to Luke, for example, where we see John as a social reformer or in Mark where, to my eyes, the clearest context is that these are earthly, lived sins that one is repenting for.

As for John's movement, I think it's clearest from Josephus:

[John] who was a good man, and commanded the Jews irate, both as to righteousness towards one another, and piety towards God, and so to come to baptism; for that the washing [with water] would be acceptable to him, if they made use of it, not in order to the putting away [or the remission] of some sins [only], but for the purification of the body; supposing still that the soul was thoroughly purified beforehand by righteousness.

So John's movement was one about personal transformation--perhaps in preparation for a coming future event (like Jesus, or a day of judgement, or whatever), perhaps not. But I don't think you should assume it was only possible to administer once. It should only be administered once, because there was supposed to be a spiritual cleansing along with the bodily cleansing (perhaps relate this to Jesus's, "now go and sin no more"), but there isn't a hard limit implied anywhere in the texts if the spiritual purification faltered and needed to be reperformed. Indeed, Mandaeans, who claim spiritual descent from John the Baptist (historically, this lineage is questioned) but reject Jesus, continue to perform ritual baptisms, and not only once.