Depends on the company for cover letters really. Some have it as a formality and some take them seriously. As a rule of thumb, don't double up information in an exact manner. Don't take what's on your resume and copy almost exactly onto a cover letter.
Just start out with saying who you are and stuff like that, and then move into why you would be good for the position and what you bring to the table for the company. You might mention some work you've done before, which MIGHT be on your resume, but more of the stuff that directly relates to the job you're applying to. You might be able to expand on a point more than just one sentence (like your resume says "-Worked on ___________" let's say (not a good example) and you might expand on that point in your cover letter for how it relates to the job you're seeking)...but also write more about how you're proactive (if you are), like working with people, strive to get things done, etc etc etc. Stuff like that..soft skills that might not be explained on your resume 100% well.