Kommst du mit uns mit?

English Translation

Are you coming with us?

I know that German syntax can be a little bit “interesting”, but that double “mit” cannot possibly be right… er, can it?

It is.

The first “mit” belongs to “mit uns” - with us
The second is from the separable verb “mitkommen” - to come along. Basically it corresponds to the “along”

So together it’s “Are you coming along with us?”