The mergeAll option allows to create two Debug instance controlling the same file.
Even if a common PrintWriter is used, some local variable could be in conflict. This is the case for the variable "lastRotation", used to create a new file (shared between the two instances) if the rotation is needed.
A correction will be to avoid the possibility to have two (or more) Debug instance if the mergeAll option is used. The DebugProvider is a good place for doing that : it can check the mergeAll options and return the same Debug instance if the mergeAll option is ON.