flyspell-mark-duplications-flag

flyspell-mark-duplications-flag is a variable defined in `flyspell.el'.
Its value is

t

Documentation:
Non-nil means Flyspell reports a repeated word as an error.
See `flyspell-mark-duplications-exceptions' to add exceptions to this rule.
Detection of repeated words is not implemented in
"large" regions; see variable `flyspell-large-region'.

You can customize this variable.