Using grep to find duplicate characters in a word in bash

[LHUHCD]

Is there a regular expression that can find duplicate characters in a word?  

For example, for five-letter words, we would use..... 
     grep "^.....$" 
.....to find any five character word
 
How can we, at least, find five-letter words with duplicate letters in the
word using grep in bash?

ample - no
apple - yes
pills - yes
civic - no (two pairs of duplicates)
alana - no (includes "triplicate")

At the very least, I am looking for a grep that finds words with at least a
duplicate letter inside the word.

Thanks!


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