Handle duplicate attribute and TLV entries specially.

For attributes, handle them the same way we handle duplicate vendors.

For TLVs, ignore duplicates; they shouldn't happen.

Change-Id: Ie968478c40a9b7848fa8ea25b144eda8656e5874
Reviewed-on: https://code.wireshark.org/review/5268
Reviewed-by: Guy Harris <guy@alum.mit.edu>