I have noticed when updating computers on my network that the Update Accelerator seems to be re-downloading any names that have special characters in the download name.
I have updated several of the same machines and they use the cache version until they hit a name with special characters in it.
Ubuntu uses ~ in their file names a lot and these don't get used by the cache and they get downloaded again.
Here are some examples from the cache.log file
Linux DLSOURCE http://au.archive.ubuntu.com/ubuntu/pool/main/u/unity-control-center/unity-control-center_15.04.0%2b16.04.20170214-0ubuntu2_amd64.deb
Linux DLSOURCE http://au.archive.ubuntu.com/ubuntu/pool/main/g/gcc-5/g%2b%2b-5_5.4.0-6ubuntu1%7e16.04.5_amd64.deb
It seems to function as designed when the names don't have special characters in them.
I am using IPFire 2.19 (i586) - Core Update 117
There a thread in forum which faces the same problem.
I'll supply a resolution soon.
*** This bug has been marked as a duplicate of bug 10344 ***