Double encryption when using encrypted Time Machine on MacOS Extended encrypted?

When I create a backup partition on an external USB drive formatted as Mac OS Extended (Journaled, Encrypted) and then use Time Machine to backup to this partition with Time machine's encryption enabled, are the files encrypted twice on different levels (application level and file system level)?

Right now I am very confused because Time Machine is "Decrypting ..." my drive, which should be encrypted due to the file system.

When use select Encryption in Time Machine it is actually the disk that is formatted as Encrypted - the same way you yourself could format an external USB drive as Encrypted. The files are not individually encrypted on two different levels.