I am not sure if you mean you are getting additional LightSelects, one of which duplicates the spline - that is, one LightSelect contains the Environment checked and the spline, and there's another LightSelect with just the spline.
If so, don't forget you can manually adjust LightSelect settings to specify what lights / objects they include. You can either build the LightSelects entirely manually, or use the automatic creation method and then edit the result - e.g. simply delete the spline from the LightSelect that controls the Environment, or delete the other LightSelect that contains just the spline.
Alternatively, if you mean the spline only appears in one LightSelect on its own (with Environment not checked in that LightSelect), and the LightSelect for the Environment does not also contain the spline but adjusting the Environment LightSelect in LightMix causes changes to the spline as well (even though it's not in that LightSelect), let us know. If that's the case, is the spline material related to the Environment material at all?
Some screen shots of the LightSelects, and of the results of changes, could be useful to know what it is you are experiencing (or the scene itself even).
Thanks!