Double Comment Field
When opening the modal, a comment field appears, but when i click on reply on another comment, another one pops up. Should only be one.
I disagree with you here, this is the right thing to happen.
The first comment box when you open a modal (bottom on your screenshot) is replying to the top level / original comment.
The second comment box (first on your screenshot) when you click reply to an existing comment is commenting on the threaded comment conversation.
That makes most sense so that people can see the level that they're replying at.