Yes, I've had the same problem, and have a sort-of solution. I frequently write original stories with TextWrangler, and then transfer them to the WordPress editor. After much experimentation, here's my technique to copy plain text to the WordPress post editor: copy the text from TextWrangler to the clipboard, go to WordPress and click Add New Page, switch to HTML mode in the WordPress editor, paste the text into the editor window, and immediately click the Save Draft button. I repeat, immediately. Once the text is saved, you can switch back to Visual mode and begin doing a final edit on the text. If you paste the text while in Visual mode, or paste while in HTML mode, switch to Visual mode and then save, your text will end up with all sorts of formatting code and duplicate text. The trick is to paste into HTML, then save.