Moved to My Account Online. It says the number has already been used so have you installed it previously? If so merely go your online account and re-download from there.
Failing that, contact Customer Service by phone or online chat. See the link below.
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I would suggest to clear all Temporary files (See here ) and retry the Install by downloading the fresh Installer from McAfee's My Account Download page.
Please note that this Serial Number changes for every download.