[InputBuilder] Make sure users can't add the same feature to the same nesting level of an LHS twice.
[InputBuilder] Make sure users can't add the same feature to the same
nesting level of an LHS twice.

This functionality might be re-enabled when adding support for allowing
multiple OR'ed values for atomic features, and automatic merging of
substructures for complex features.

itsjeyd/ODE,itsjeyd/ODE,itsjeyd/ODE,itsjeyd/ODE