Can you add duplicates fields in a structured section's GUI?
An entry type can only use a field once, regardless the tab. But you can reuse the field in other entry types of that same section or reuse it in other sections.

Keep in mind, that you (currently [*]) always have the same field settings shared with each instance you use it. Meaning that you'll always end up with the same field name and instructions, which sometimes (I'd say often) is a deal breaker for reusability.

[*]: if I remember this right, I already made P&amp;T aware of this and made a feature request