Time Machine 2e disk Issue, High Sierra 10.13.1 ...

I am using a second external disk(B) to make a second Time Machine backup of my 1Tb+ space residing on internal 1Tb iMac Fusion disk(X) and an external Lacie 3Tb Thunderbolt2(Y). Alternatively it uses the one(B) then the other disk(A) to backup; perfect! But since High Sierra, after a number of successful incremental backups on both disks(A) and (B), the second one (B) backs up the total 1Tb+ space(X)+(Y), to be backed up, a second time without anything fundamentally changed and so using up all space on the second BU disk(B). I stopped that process and re-initialised disk(B) thoroughly and re-started the process with adding the empty(B) again to Time Machine. After a few successful double incremental backups on (A) and (B) the problem resurfaces and the total space is being back-up two times on the second backup disk(B) without a reason that I can see, while (A) stays normal taking incremental BU's. The situation is troubling my confidence in Time Machine as my important backup basis!

Anyone experiencing the same or having a solution to this?

Thanks in advance!

Using iMac27inch macOS 10.13.1, (A): Lacie 6Tb Thunderbolt2, (B): WD MyBook 2Tb