I think the answer is
a plagiarized question on PuzzlingSE
You know I think I've been here before,
Every now and again new users join and add cliched riddles and puzzles without any attribution.
And very soon I am shown the door.
Emrakul brings down his hammer on these questions and off they go into the garbage bin.
Someone new brought me here,
As mentioned, such questions are often brought up by new users on PuzzlingSE.
To another place I will steer.
Their clues can very easily lead us to the actual source with a simple Google search.
Little can be done to fix,
We can downvote and flag repeatedly, and Emrakul can delete them but little can be done to fix this issue permanently.
I shouldn't even be in the mix.
In an ideal scenario, we should never have to deal with such plagiarized questions.
I must already be known,
Most of the plagiarized riddles and puzzles are already known to the expert puzzlers here.
Who am I? Not really alone.
You are a plagiarized question, and there can be several of you hanging around on PuzzlingSE even after Emrakul and the community's vigilante efforts.
This answer is inspired by this meta post.