This is fascinating (to me, at least).
The Periplus of the Erythraean Sea (from the first century, common era) lists a trading port named Rhapta near what is now the straights of Mozambique. The exact location is under dispute, but it is generally thought to be around the modern city of Dar es Salaam.
In the 10th century, a group of Arab traders made their way down the African coast and set up shop on an island called Kilwa. The city they built on this island became one of the largest trading ports on the east African coast. It’s probably not terribly surprising to compare the geography and notice that both trading ports were relatively close to one another - Kilwa is just down the coast from Dar es Salaam.
What I find fascinating is that two seperate and distinct port cities were established in more or less the same area. I doubt that Rhapta was actually on Kilwa island, but it was a large enough destination to be noted on a first century document. And it would not be much of a stretch to imagine that conditions in both ports were more or less the same, despite the thousand year time span that seperated the two.