Remove repeating characters from words

You should do it without reduce or regexps:

>>> s = 'hhaaaaapppppyyy'
>>> ''.join(['' if i>1 and e==s[i-2] else e for i,e in enumerate(s)])
'haappyy'

The number of repetitions are hardcoded to >1 and -2 above. The general case:

>>> reps = 1
>>> ''.join(['' if i>reps-1 and e==s[i-reps] else e for i,e in enumerate(s)])
'hapy'

It can be done using regular expressions:

>>> import re
>>> re.sub(r'(.)\1+', r'\1\1', "haaaaapppppyyy")     
'haappyy'

(.)\1+ repleaces any character (.) followed by one or more of the same character (because of the backref \1 it must be the same) by twice the character.

You can squash multiple occurrences of letters with itertools.groupby:

>>> ''.join(c for c, _ in groupby("haaaaapppppyyy"))
'hapy'

Similarly, you can get haappyy from groupby with

>>> ''.join(''.join(s)[:2] for _, s in groupby("haaaaapppppyyy"))
'haappyy'