All you have to do is Google the words "Dylan" and "plagiarism" together and you get 1,460,000 hits. Here is one example, revealed in 2009.
A poem supposedly written by Bob Dylan as a teenager is an old country song, it emerged yesterday. The bombshell came just 24 hours after the original handwritten version of Little Buddy was put up for auction by Christie's. It had been touted as an example of Dylan's "brilliance even as a 16-yearold". But, as a country fan pointed out, most of the words were from a song by Canadian Hank Snow, whose song was recorded in 1948 - 11 years before Dylan submitted his poem to a Jewish summer camp newspaper in Wisconsin.
I have heard about Dylan using other musicians and their lyrics as long as I can remember, and that is a looooooooooooooooooooooong time. I first heard it in regard to Woody Guthrie. Dylan's "Song To Woody" on his first album is attributed to Bob Dylan, but uses the music to Woody Guthrie's "1913 Massacre" without any attribution. Also, "With God On Our Side" does the same with "The Patriot Game".
In late 1961, Bob Dylan recorded "House of the Rising Sun" for his first album, Bob Dylan, released in March 1962. Dylan claims a writer's credit for the song. Actually, the authorship of "The House of the Rising Sun" is uncertain. Some musicologists say that it is based on the tradition of broadside ballads such as the "Unfortunate Rake" of the 18th century which were taken to America by early settlers. Alan Price of the Animals has claimed that the song was originally a sixteenth-century English folk song about a Soho brothel, and that English emigrants took the song to America where it was adapted to its later New Orleans setting.
And so on....