Why know it is Python when....

it-is-pythonic.py

value_list = [
    {
        'name': 'a',
        'value': '1',
    },
    {
        'name': 'a',
        'value': '2',
    },
    {
        'name': 'b',
        'value': '10',
    },
    {
        'name': 'b',
        'value': '11',
    },
    {
        'name': 'c',
        'value': '100',
    },
]

DELIMITER_SYMBOL = ','

# you start with
result_dict = {}
processed_slugs = []
for item in value_list:
    if item['name'] not in result_dict:
        result_dict[item['name']] = item['value']
        processed_slugs.append(item['value'])
        continue
    if item['value'] not in processed_slugs:
        result_dict[item['name']] += f'{DELIMITER_SYMBOL}{item["value"]}'
        processed_slugs.append(item['value'])
print(result_dict)

# then you go
result_dict = {}
all_names = {item['name'] for item in value_list}
for item in sorted(all_names):
    filtered_list = list(filter(lambda x: x['name'] == item, value_list))
    slugs_list = list({single_item['value'] for single_item in filtered_list})
    result_dict[item] = DELIMITER_SYMBOL.join(sorted(slugs_list))
print(result_dict)

# and you finish with
all_names = {item['name'] for item in value_list}
result_dict = {
    item: DELIMITER_SYMBOL.join(
        sorted(list({single_item['value'] for single_item in value_list if single_item['name'] == item})))
    for item in sorted(all_names)
}
print(result_dict)