Allright, in that case I think it is more an accounting error than a duplication, but we'll have to check that. If you go to 'About this Mac'/storage/' you'll see how much empty space there is on your drive. Note that number and then go to 'Manage storage' and 'iOS files' and delete the backup. Give your system some time for the dust to settle (I find that it sometimes takes a while before the grand total numbers change) and check how much the free storage has changed, it should have gone up by 102.78 Gb. Also, check if your applications size is still showing 108.61 Gb, it should be down to something realistic for your system. I suspect that the files are not really duplicated but just stored in such a way that they are counted twice.
You can also use the method described here:
https://support.apple.com/en-gb/HT204215 to locate and delete your phone backups. This way you might be able to see whether there is a single backup file or more than one.