Bug 17913: Remove possible duplicates in strict merge mode

Bug 17913: Remove possible duplicates in strict merge mode

Since strict mode does not allow additional subfields that would make
identical fields linked to the same authority different, there is no
need to keep them while merging.

We achieve this goal by simply:
[1] Count the number of same fields linked to mergefrom in strict mode to
    eliminate duplicates.
[2] Replaces the if-statement on auth_number by a next. (Tidy follows.)

Test plan:
Run t/db_dependent/Authorities/Merge.t

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>