Randomly Misspell a Word
Pip, 38 27 bytes

a@:`(.)\1*`YRR#aa@y@0X:RR3a


This was fun--got to use Pip's regex and mutable string capabilities, which I hadn't pulled out in a while. Takes input via command-line argument.

Explanation:

a@:`(.)\1*`                  Split a into runs of identical chars using regex match
           YRR#a             Yank randrange(len(a)) into y (randomly choosing one such run)
                a@y@0        Take the first character of that run
                     X:RR3   Modify in place, string-multiplying by randrange(3):
                               If RR3 is 0, character is deleted
                               If RR3 is 1, no change
                               If RR3 is 2, character is duplicated
                          a  Output the modified a


Try it online!