"I have known him for a long time."
Some verbs are composed of a verb base character with a noun base character, e.g. 骑马，打球，吃饭. With these verbs we repeat the verb base character 骑，打，吃.
Some verbs are formed by 2 verb base characters of identical or similar meaning, e.g. 认识，观察，沐浴. With these verbs we usually do not repeat like that, and if we do we repeat both characters e.g. 他观察野生动物已经(观察了)二十年了. Normally it would be regarded as clumsy and unnecessary.