Xpath to find duplicate node values

Question

I have following xml file:

<Dictionary>
   <words>
       <word>AM</word>       
   </words>
   <words>
       <word>AN</word>
   </words>
   <words>
       <word>AN</word>
   </words>
</Dictionary>

I need xpath to find duplicate word nodes like in above "AN" is duplicate. I tried following from web search but it did not work:

/*/*[index-of(/*/*/word(), word())[2]]

Answer 1

Select word that exists the same word before

/Dictionary/words/word[.= ../preceding-sibling::words/word]

Answer 2

With following axis:

//words/word[. = ./following::word]

https://www.w3.org/TR/1999/REC-xpath-19991116/#axes