Heather Thomson, founder of shapewear brand Yummie Tummie, has fired back at Spanx founder Sara Blakely after Blakely produced tank tops similar to ones Thomson says she’d patented. But Blakely’s filed a complaint in the Northern District Court of Georgia, arguing Thomson’s trademarks on the tanks aren’t valid. And, in turn, Thomson’s written an open letter to Blakely, which she planned to post on her Web site last night. “Until now, I have had only the utmost respect for the growth of your Spanx business,” the letter reads. “Unfortunately rather than showing me the same respect, you and Spanx have deliberately and willfully infringed on my patented inventions.” A Spanx rep didn’t get back to us. Stay tuned.