please see the heapdump at https://netbeans.org/projects/performance/downloads/download/heapdump_232437.7z there are two instances of org.openide.util.lookup.implspi.ActiveQueue$Daemon
this state was achieved through repeated builds of netbeans modules like Core, Lookup API, etc.
there is interesting thread dump attached to issue #228424 - there are 19 active reference queue daemons there.maybe it would be worthwile working wit alexander on reproducing this issue...
The more I think about the issue, the more I believe this has already been resolved. Btw. there is proposal to make this work on JDK without the reflection hacks: https://bugs.openjdk.java.net/browse/JDK-8051843 - support welcomed.
*** This bug has been marked as a duplicate of bug 206621 ***