Re: Turning down recommendation
Dear Ms. Smith/Mr. Jones,
Thank you so much for your offer to write me a letter of recommendation. As it happens, I have already completed the application that required the letter, so I do not need a letter at this time. I hope I may keep you in mind for a future reference, if the situation arises. I truly appreciate your generosity.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.