Just annexed this territory and it has 2 Nazca Lines and 2 Knyaz Libraries.

Is there an ingame bonus that allows you to do this, or did I encounter a bug? (no other territories seem to have double districts)

PS: playing the Official Endless Mod

OEM - Argentinians Turn 167 - Double district.ctr

It's a Natural Wonder thing if I'm not mistaken.

The Nazca legacy trait lets you build two EQs (per era!) in territories that contain natural wonders.  So it's because that territory was owned by the Nazca and contains Salto Angel.

The wonder in the territory is Salto Angel (5 science, 3 influence on commons, 10 faith on city center).

Pink does not own other natural wonders.

And I can't build copies in the territories where I have natural wonders.

Oh, I didn't know about the Nazca trait. Thanks RedSirus

Yes Legacy Trait of the Nazca.

If you want to go for a faith/influence run, they are key. They add faith/influence to every EQ, which is going to be the quarter you build the most so why not. Then there's the bonus that you get double all EQs - ie every era from Nazca on - on any territory that has a natural wonder. You will be bombing faith and influence. I guarantee it ;P