pyMessageDict

data = {
    'user_id': [1, 1, 2, 2, 1, 1, 1, 2, 2, 1, 1],
    'message_text': [
        'Hi, How are you?', 'I hope you remember me', 'I am fine.', 'how can I forgot you', 'that is great 🙂',
        'so what is the plan?', 'I hope you will not back up',
        'off course not', 'let me set a timing for it', 'ohh nice', 'thanks'
    ]
}

output = {"user_id": [], "message_text": []}

last_id = -1
temp = []
for x, id in enumerate(data["user_id"] + [9]):
    if id != last_id:
        if temp:
            output["user_id"].append(last_id)
            output["message_text"].append("\n".join(temp))
        temp = []
    if id == 9:
        break
    temp.append(data["message_text"][x])
    last_id = id

print(output)

# Result: {'user_id': [1, 2, 1, 2, 1], 'message_text': ['Hi, How are you?\nI hope you remember me',
# 'I am fine.\nhow can I forgot you', 'that is great 🙂\nso what is the plan?\nI hope you will not back up',
# 'off course not\nlet me set a timing for it', 'ohh nice\nthanks']}