Duke Nukem is downloadable if you purchased it before it got pulled off the shops but it's no longer supported.
It was removed due to Gearbox suing the crap out of anyone releasing Duke Nukem games that weren't them, who later released the World Tour Edition which has not been released on PS Vita.
And yeah Merczone has been a go-to handheld shooter for me for years. And yeah, love that people are still playing it.