I've seen in the doc which priority is applied when writing a tag that can be defined in several places (eg. Exif and XMP)
But what about display of duplicate tags at reading (when -a is not specified, of course) ?
If the tag has the same value everywhere, no importance, but in the case the tag has different values ? In such a case, which one preferred, and displayed ?
Usually it's last in, first out. And Composite tags are created last, so if a Composite tag is a duplicate, it will be the one listed.
An example of this are the GPS tags. The Composite GPS tags are more complete than the EXIF ones, as they combine the coordinates and reference directions (two separate tags) into a single tag to for more complete listing and copying.