Another fellow applicant here so take my advice with a grain of salt. From my understanding, a master's can definitely help, especially if your undergraduate performance was less than stellar; however, I know there are others on here that have gotten into phd programs without a master's, so it is not absolutely necessary. I wonder what percentage of students possess master's when they enroll? I'm currently completing a second master's, but I know that is not the norm. Ultimately, I think it all depends on how well you performed as a undergraduate and how much promise you would show as a prospective graduate student. I think that by already having a master's you can help allay some of those fears. Of course, picking up a master's isn't exactly cheap. Therefore, if you feel you will be a competitive candidate with only a undergraduate degree (strong LORs, test scores, GPA, research experience, etc) then you might as well give it a shot. If that doesn't work out, pick up a master's and try again.