Virtually every resume I see - especially those done by other professional resume writers - follows a format like this:
There are some variations, but that's essentially the norm. If you use a template (which I don't recommend), you may even have an awards section at the bottom (way, way too important to be tacked on to the end).
The problem with the top section, after name and contact info, is that the summary and core competencies section often say the same thing. Most summaries I see go on and on about how passionate, skilled, and talented the person is. Rather than actually summarizing the person's qualifications and background, it describes what the person believes they're good at.
What does the Core Competencies section then do? The same thing. It goes on and on about what the person believes they're good at. The only difference is the Core Competencies section is in list form and the summary is in paragraph form. That's it. Otherwise, it's pretty much the same.
So why do people do this?
One, because that's what we're used to seeing. So often when people write a resume they think back to what they've seen on other resumes, assume that's the way it has to be done, and then they copy that format. Most people don't stop to ask why - they just assume it's "supposed" to be done this way, and they do it.
Second, they use the Core Competencies section to shove more keywords into their resume. Keywords have been all the rage for several years now, to the point where they're viewed as much more important than they really are. And the Core Competencies section makes a great place to add more and more.
But the truth is you don't need to start off your resume with a summary that says what your skills are, and then follow that up with another list of skills. It's just not necessary. All it does is push your work history (the #1 thing that recruiters want to see) further down the resume.
I've been writing resumes professionally since 2005 and I can't recall the last time I used a Core Competencies section. Maybe I never have - I don't remember. I do occasionally list a few skills in a summary section (not dozens), but I don't focus on that too much. If you have a summary (it's not always needed), it should truly summarize your qualifications, not just list a bunch of random skills.
In most cases, the bulk of your resume should be your work history - where you've worked, what positions you've held (and when), what you've done, and how well you did it.
Don't waste too much space on what you believe you're good at. Show what you've accomplished and let the reader decide.