disallow same-named columns, unchecked replacement in Table

disallow same-named columns, unchecked replacement in Table

Fixed issue where table reflection using :paramref:`.Table.extend_existing`
would fail to deduplicate a same-named column if the existing
:class:`.Table` used a separate key. The
:paramref:`.Table.autoload_replace` parameter would allow the column to be
skipped but under no circumstances should a :class:`.Table` ever have the
same-named column twice.

Additionally, changed deprecation warnings to exceptions
as were implemented in I1d58c8ebe081079cb669e7ead60886ffc1b1a7f5 .

Fixes: #8925
Change-Id: I83d0f8658177a7ffbb06e01dbca91377d1a98d49