I have a word list, such as:
I want to remove letter and tagggs from it (because they have two or more of the same letters in a row). Maybe this can be done using sed?
You can use one of the regex tools that supports backreferences e.g.
sed '/\(.\)\1/d' file
grep -v '\(.\)\1' file
Change . to [[:alpha:]] if you really want to restrict it to repeated letters rather than any repeated characters.