Before recordings, this is maybe a murky area, because before the existence of music conservatories ( outside the Church) composers would study under another composer. The first few symphonies of Beethoven sound a lot like Haydn because, well, he studied with Haydn and that was somewhat expected....

There's also the question of style. Boito liked, and wrote one opera, that sounded a lot like Wagner. There were differences, but if you didn't like Wagner ( and there were plenty of Verdi-fans in Italy who did not) you didn't like it.

There's also the matter of writing variations on themes. Sometimes the theme was known- like Packington's Pound or Espangoletta- and the variations could be formulaic enough to be almost interchangeable. Indeed, the performer was supposed to be able to improvise them, and you can imagine some were more imaginative than others.

But today when someone like John Williams really "channels" Holst for his Star Wars music, people shrug it off. It's open, for everyone to see. Call it homage.

Is your question "Did classical composers rip off other people's music?" Because yeah, that happened all the time.

But if instead your question is "Was there some accusation of legal theft akin to a modern copyright infringement against a classical composer?" Then you're going to have a very different answer, because plagiarism isn't viewed today the way it was a few hundred years ago.