Returns the custom labels that are shared by more than one atom.
Atoms without a custom label are ignored, so a molecule with no custom labels,
or with only unique ones, yields an empty array.
Parameters
molecule: Molecule
The molecule whose atom custom labels should be checked.
Returns string[]
The deduplicated list of non-unique custom labels, in first-seen
order. Empty when every custom label is unique.
Returns the custom labels that are shared by more than one atom. Atoms without a custom label are ignored, so a molecule with no custom labels, or with only unique ones, yields an empty array.