Description:
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IteratorIterator and derived classes like FilterIterator, RegexIterator etc. are not cloneable, because the implementation for spl_dual_it.clone_obj is missing.

This is issue, especially because Iterator is clonable, IteratorIterator is derived from Iterator and is not clonable, thereby violating the liskov substitution principle.

Test script:
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<?php

$arrayIterator = new ArrayIterator(array(1,2,3));
$it1 = new IteratorIterator($arrayIterator);
$it2 = clone $it1;

Actual result:
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PHP Fatal error:  Trying to clone an uncloneable object of class IteratorIterator