exc_inspect: dup file and check mesg only once

```ruby
begin
  fail ArgumentError, 'wrong number of arguments'
rescue => e
  p e, e
end
```
```bash
test.rb:2: undefined method 'fail' for main (NoMethodError)
test.rb:2: undefined method 'fail' for main (NoMethodError):2: undefined method
'fail' for main (NoMethodError)
```