Если данные взяты из строки JSON, это также можно сделать во время синтаксического анализа:
var result = {}, json = '{"stats":[{"statsFields":{"duo":2,"duoM":2,"duoW":2,"kdr":2,"solo":2,"soloM":2,"soloW":2,"squad":2,"squadM":2,"squadW":2,"total":2,"totalM":1,"totalW":2,"winP":2},"_id":"5cc283ba7b752f322ce26168"},{"statsFields":{"duo":2,"duoM":2,"duoW":2,"kdr":2,"solo":2,"soloM":2,"soloW":2,"squad":2,"squadM":2,"squadW":2,"total":2,"totalM":1,"totalW":2,"winP":2},"_id":"5cc284cd7b752f322ce26169"}]}'
JSON.parse(json, (k, v) => v.toFixed && (result[k] = result[k] + v || v))
console.log( result )