When two relationship of the same type, from the same entity, to the same entity, and with the same attributes, are currently allowed if and only if they have two different date periods (set through begin date, end date, ended flag).

This could be improved by detecting overlapping date periods and issuing a warning or an error in such case.

Both external links editor and relationship editor can benefit of that. These are two distinct pieces of code, so feel free to split in two tickets if not implemented at the same time.