  We show that any preadditive infinity category with duality gives rise to a
direct sum hermitian K-theory spectrum. This assignment is lax symmetric
monoidal, thereby producing E-infinity ring spectra from preadditive symmetric
monoidal infinity categories with duality. To have examples of preadditive
symmetric monoidal infinity categories with duality we show that any
preadditive symmetric monoidal infinity category, in which every object admits
a dual, carries a canonical duality. Moreover we classify and twist the
dualities in various ways and apply our definitions for example to finitely
generated projective modules over E-infinity ring spectra.
