Description:
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Unserializing an object after changing some of its class properties' 
from public to protected results in properties present in both states. 

(As a workaround, migration code can be written using get_object_vars() 
to update the a protected property from the corresponding public version 
within a __wakeup() call.)

Reproduce code:
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(It's difficult to write reproduce code for this, so I hope the following step-by-steps are OK)

1. Object "John" of class "Person" stored in serialized form has property "age" with public visibility.
2. Change visibility of property "age" in class definition to "protected".
3. Unserialize "John". The property "age" will be present in both public and protected states.
4. Attempting to access the "age" property directly correctly returns the value stored in the protected version.
5. Using get_object_vars() returns the value stored in the public version.

Expected result:
----------------
Changes in property visibility should migrate the values gracefully upon 
unserialization. Properties by any given name should only exist once, 
but the current behavior conflicts with that.

Actual result:
--------------
Both versions of a property (public and protected) exist in unserialized 
object.