python list of dictionaries find duplicates based on value

You can try:

import operator, itertools

key = operator.itemgetter('id')

b = [{'id': x, 'desc': [d['desc'] for d in y]} 
     for x, y in itertools.groupby(sorted(a, key=key), key=key)]

It is better to keep the "desc" values as lists everywhere even if they contain a single element only. This way you can do

for d in b:
    print d['id']
    for desc in d['desc']:
        print desc

This would work for strings too, just returning individual characters, which is not what you want.

And now the solution giving you a list of dicts of lists:

a =[{'id': 1,'desc': 'smth'},{'id': 2,'desc': 'smthelse'},{'id': 1,'desc': 'smthelse2'},{'id': 1,'desc': 'smthelse3'}]

c = {}
for d in a:
    c.setdefault(d['id'], []).append(d['desc'])
b = [{'id': k, 'desc': v} for k,v in c.iteritems()]

b is now:

[{'desc': ['smth', 'smthelse2', 'smthelse3'], 'id': 1},
 {'desc': ['smthelse'], 'id': 2}]