[electrohphysiology_browser] Prevent ability to add duplicate HED tags

[jeffersoncasimir]

For compliance with the HED spec, duplicate tags should not be permitted at the same nesting level.

Extra attention is needed for tag grouping actions to ensure compliance with the above.

[christinerogers]

Deleted this from the Loris 26 Release Testing project since it can't be fixed until after the Release (it's too big) -- hope that's ok to take it off the bug-fix table for now