BUG-2627: do not duplicate descriptions

This class effectively creates a repeateable annotation. Unfortunately
in Java 8, we get twice as many annotations, which leads to duplicate
entries. Instantiate an intermediate Set, so we suppress potential
duplicates.

Change-Id: I0377d439260c2b3793c972c73373f5f6c6a7f119
Signed-off-by: Robert Varga <rovarga@cisco.com>