The language I am creating is derived from East Norse and heavily influenced by Slavic. It has a quite complex grammatical structure - for example, it has not only taken over the two additional cases from Russian but also developed several own ones.
One unique aspect is the dual noun form, which is avaliable for describing things that usually occur in complementary pairs. There are dual-only nouns for things that only occur in the described pairs (such as a romantic couple) and dual forms of ordinary nouns in “natural” couple forms (a pair of shoes, the poles of a magnet, my eyes - not two houses or a couple of road signs). Some features are either identic to either singular and plural. A part of the pronouns - most notably all second-person pronouns - are however unique (somewhat similar to the dialect of Bavarian spoken in Munich).
I am wondering how I can “trace back” such a development. According to my knowledge, (correct me if I am blatantly wrong) neither East Norse nor Old East Slavic had the aforementioned features, and just “inventing it by accident somewhere around 1100” would sound like a too handwaved explanation.