Tài liệu PL/SQL User’s Guide and Reference phần 5

You cannot declare an exception twice in the same block. You can, however, declare
the same exception in two different blocks.
Exceptions declared in a block are considered local to that block and global to all its
sub-blocks. Because a block can reference only local or global exceptions, enclosing
blocks cannot reference exceptions declared in a sub-block.
If you redeclare a global exception in a sub-block, the local declaration prevails. So,
the sub-block cannot reference the global exception unless it was declared in a
labeled block, in which case the following syntax is valid