Does contemplating performing Prapatti a second time create the ‘Brahmastra Nyayam’? Or does that flaw actually occur only if it is performed a second time?

The Brahmastra dosha (flaw) occurs only if you perform Prapatti a second time. One should not do that. Even thinking of doing it is wrong. Since the Brahmastra flaw will occur if you think of it and then do it, it is best not to think of doing it.