The emulation of variadic templates used by the Log class relies on the fact that __NO_TAG terminates a tag sequence. Log<logging, __NO_TAG> is the log for the {logging} tagset, for example. GCTraceTimer appends tagsets with a 'start' tag for the timer start logging. While doing so it instantiates Log templates where other tags follow the __NO_TAGS tag, thus creating what looks like duplicate tagsets.

This affects the listing of tag sets in -Xlog:logging=trace (see for example -Xlog:logging=trace,gc).