Make the strings in the stringlist-to-be-joined unique, which matches
actual use-cases better than joining a list of identical strings,
especially with QString's implicit sharing, if it's copies of the same
QString, it's sharing the underlying data.
Pick-to: 6.5 6.2 5.15
Task-number: QTBUG-116859
Change-Id: I1da93885e938045322ba8337df5e4e96985f892f
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit e1dcc858b2a0ac2b359cdbf7e07e70c1237daef0)
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
e1fe8963a7