Answer:
Repeating too many details from the resumé
Step-by-step explanation:
A cover letter should increase the applicant's chances of getting a job position, so it should be well-written, concise, it should subtly show the applicant's personality and present their relevant experience and aptitudes to the job position, as well as contact information and it may or may not address the recipient of the letter by name.
This letter should be easy to read and should invite the interviewer to keep reading more, so a "must" is to avoid redundancies or repetitions of many details from the resumé, as for it would bore the interviewer out, it would be an indication of the applicant's poor writing skills and ability to synthesize and the letter would be easily discarded.