In Duisburg and Düsseldorf I have heard people talking a mixture of German and Dutch which really confused me! Can anyone please explain how similar to Dutch this so called Low German language is?
Stadard Dutch is different from Low German (Niederdeutsch) in being Low Franconian (Niederfränkisch). There is a Low German dialect in the Netherlands, Gronings, that is a recognised regional language. But when you look at this map on Wikimedia Commons you can see that the Low Franconian dialects also have some footing in Germany, namely Niederrheinisch (Low Rhenanian) coded with the number 12 in the image. Their range includes Duisburg and Düsseldorf. So the native dialect of those cities is indeed closer to standard Dutch than any other dialect spoken in Germany.
But be aware that High German acts as dachsprache upon the Low Rhenanian dialect drawing it away from standard Dutch over time.