You know you have to tailor both your resume and your cover letter to every job you apply to, but it's easy to do so much that they overlap. They should complement one another instead, to tell one story about your skills. Before you send in your application, make sure they fit together nicely. Image from bearstache.
It's easy to repeat your work history or skills on your resume when writing your cover letter, as you want to share those things to show why you're a good fit for the job. But your cover letter should complement your resume, not duplicate it. Before sending in your application, compare the two and make sure they're a complete story of your skills and background, with one adding to the other, but both standing well alone. Cut out any repetition and use the space in your cover letter to share anecdotes or other specific examples of your skills in your resume that reflect why you're a strong candidate.