Reformatted Windows HDD shows up twice in Disk Utility
They aren’t two separate drives, they are two different partitions on the same drive.  You need to repartition the drive so that there’s only one partition.
If you issue the command:
% diskutil partitionDisk /dev/diskX 1 GPT JHFS+ “New Name” 100%

Will wipe the entire drive with a single partition using a GPT partition scheme and format it with the Journaled HSF+ (macOS) format.
If you issue the command diskutil partitionDisk without any arguments, you will get the following help info:
diskutil partitionDisk MountPoint|DiskIdentifier|DeviceNode
        [numberOfPartitions] [APM[Format]|MBR[Format]|GPT[Format]]
        [part1Format part1Name part1Size part2Format part2Name part2Size
         part3Format part3Name part3Size ...]