Repetition is a term that is used by the author to talk about the struggles that migrants face coming into a new country and trying to process their past and assimilate. This is shown most clearly in the characters of the Iqbal family, who, Smith argues, are doomed to endlessly repeat their migration in their minds and they struggle to adapt to their lives in a country that is so different from their native Bangladesh:
A trauma is something one repeats and repeats, after all, and this is the tragedy of the Iqbals--that they can't help but reenact the dash they once made from one land to another, from one faith to another, from one brown mother country into the pale, freckled arms of an imperial sovereign.
For the Iqbals, their migration meant a change of land, religion and ethnicity, and the resulting trauma they suffer is something that is repeated again and again with each passing day as they struggle to assimilate and to accept and to be accepted in their new "home." Note how Smith refers to this repetition exclusively in the context of migration later on in the text:
Because immigrants have always been particularly prone to repetition--it's something to do with that experience of moving from West to East or East to West or from island to island. Even when you arrive, you're still going back and forth; your children are going round and round. There's no proper term for it--original sin seems too harsh; maybe original trauma would be better.
Even though a migrant may be fixed geographically in their new country, they are endlessly returning back to their old "home" in their minds as they mentally find themselves struggling to adapt and thinking of their homeland. Repetition is a word therefore that Smith uses to capture the experience of a migrant and the particular struggles they have as they try to adapt and conform in a new, alien environment. The greatest tragedy, Smith suggests, is that true assimilation is impossible. As a migrant, and even as a second generation migrant, you still do not entirely "fit."