1. When reciting the word “Echad” one must emphasize the letter “Daled”. However, one may not emphasize it too much so that it does not sound as if it is vowelized with a “Sheva” or a “Tzeirei”. (Shulchan Aruch and Mishne Berurah 61)
2. One may not double the Shema, whether one repeats the word “Shema” twice or one repeats the entire first verse twice, as it looks like he is accepting upon himself the power of two separate entities. If one did not concentrate in the first verse, when the law is that he must repeat it, if there is no one to hear him, he may repeat it out loud, but if there are other people listening, he must read it quietly. (Shulchan Aruch and Mishne Berurah ibid)
