Only a single DEG operation node `POSE_SPLINE_IK_SOLVER` should
be added in this case [ see `build_splineik_pose`, same is already done
for overlapping IK in `build_ik_pose`]
ref T82347.
Reviewers: sybren
Maniphest Tasks: T82347
Differential Revision: https://developer.blender.org/D9471