This error message "The license <serial_number> is already in use by the user <username> on the workstation <computer_name>" appears when a single license was assigned to several workstations.
This may occur when errors were made during computer cloning, creating several computers that are virtually identical. When licenses are issued, computers are identified using their SIDs and checksums of their names. If the first part of the SID of several computers and the checksum of their names are the same, they will be identified as the same computer and will receive the same license.
We recommend using Sysprep Microsoft utility for cloning computers. This will help you avoid cloning errors that create identical computers.