Clarify some error messages about duplicate things.

src/backend/access/common/reloptions.c

src/backend/access/nbtree/nbtinsert.c

src/backend/catalog/heap.c

src/backend/commands/tablecmds.c

src/test/regress/expected/alter_table.out

src/test/regress/expected/arrays.out

src/test/regress/expected/create_index.out

src/test/regress/expected/inherit.out

src/test/regress/expected/plpgsql.out

src/test/regress/expected/transactions.out

src/test/regress/expected/uuid.out

src/test/regress/output/constraints.source

src/test/regress/output/tablespace.source