Werk #2671: df: avoid duplicate items for btrfs that is mounted several times

With SLES 12 btrfs has been introduced. Here one filesystem (e.g. /dev/sda1) is mounted at a lot of directories at the same times and Check_MK had created one service for each of this - showing exactly the same stats. This has now been fixed so that only one filesystem check is being created for each btrfs device. The service in our example has the name Filesystem btrfs /dev/sda1. You need to redo service discovery or the affected hosts.

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