I'm not sure I understand the situation you're describing. Can you post a screenshot of it with the Construct Siphon Station tooltip visible?
gas giant + debris field = gas giant already has siphon station. image 1; tooltip. image 2; debri field. couldnt get entire gas giant due to attachment restrictions.
to the best of my understanding this is the issue;
im not able to make the siphon station because the gas giant "already has one" but all it has is a debris field and 3 astropolis. due to an issue described more later i always try to build the siphon stations first so as far as i can tell the only thing blocking me from building this could have been the debris field that was generated during sector creation.
ive been unable to get a repeat of this issue so i cant provide a photo of it and am probably wrong about the cause of the issue;
on a previous run i built an orbital station from the lane state union, blue mod by Protonus, first because i couldnt afford a siphon station. when i finally tried to build the siphon station i got the same tool tip message about gas giant only supporting one siphon station. which is why the siphon is super cheap now so i can build it before anything else.
[attachment deleted by admin]