In Mandarin Chinese, both
经 常generally implies a higher frequency and a more consistent pattern of occurrence. It suggests that something happens often and regularly, with a certain degree of predictability.
我 经 常 去 健 身 房 。
I often go to the gym.
常 常also expresses frequency, but it suggests a more casual or intermittent pattern. It implies that something happens frequently but not necessarily on a regular schedule.
我 常 常 在 周 末 去 逛 街 。
I often go shopping on weekends.
Overall, the difference between
他 经 常 迟 到 。
He is often late.
我 常 常 忘 记 带 钥 匙 。
I often forget to bring my keys.
她 经 常 去 旅 游 。
She often travels.
他 常 常 在 工 作 中 遇 到 挑 战 。
He often faces challenges in his work.