Yes, backstab can be triggered multiple times.
It may have been an oversight in the description text re: 'this ability can only be used once per combat'. Passive abilities don't have that text as they are persistent - and some modifier abilities can be used multiple times based on their description. The armour set abilities (in HoF) are missing the text I notice, and it should be there - think that's all I can really spot.
In book 4 I actually decided to stop labeling abilities in that way, and I actually preface the glossary with the assumption that ALL abilities can only be used once, UNLESS otherwise stated. Saves a lot of text and confusion I think