In my game I've got one Huari Fortress at Jiang's Star offering me an endless supply of "Intercept Sung Transport" missions. This doesn't seem intentional.
Here's what I did:
1. Befriended the Huari
2. Completed Huari missions in one system (in my game, I did this in Sanctuary)
3. Received invitation to Huaramarca, declined it
4. Went to another system with a Huari Fortress (in my game, I went to Jiang's Star)
5. The fortress offers an unending supply of missions. After accepting a mission to destroy a Sung Citadel and another to defend the fortress, it only offers "Intercept Sung transport". Since I'm not invited to Huaramarca again, I can take as many of these missions as I'd like from this fortress.
Did some testing. There are lots of missions available from other Huari stations but not from the fortress where the player qualified for the Huaramarca trip.
The Huaramarca mission chain is still available by returning to the 'qualifier' fortress. It can also be declined a number of times at this station and still be re-offered to the player.
Needs something like disabling further Huari missions once the player has reached the required level.
Or the Huaramarca mission chain could become available at all fortresses once the player has reached the required level. This would handle doing some missions at one fortress and then 'qualifying' at another fortress by doing more missions there.
For player info, there is only a "Destroy the Sung station" mission option if there is still a surviving Sung station in the same system. Another one doesn't get created for the mission.
And if there isn't a surviving Sung station left in the system then there is only one "Sung fleet attack" mission per fortress.
That's why there are so many Sung transport missions.