abc  123
abc  456
abc  789
xyz  456

Hash:

123->abc
456->abc
456->xyz
789->abc

##</code><code>##

xxx ==> 456->abc
xxx ==> 789->abc
yyy ==> 456->abc
yyy ==> 456->abc   (a dup)
zzz ==> 123->abc

Resulting hash

123->abc->zzz
456->abc->xxx
456->abc->yyy
456->xyz
789->abc->xxx