A stative complement is not allowed to follow the undergoer NP after a transitive verb. Therefore, a reduplication is generated to avoid a sequence of two complements. Once the verb is duplicated, aspect marker may only occur with the second verb, but not with the first verb.
他打球打了好幾年了 (t¯a d˘a qi´u d˘a le h˘ao j˘ı ni´an le/He has been playing the ball game for several years.) → grammatical
* 他打了球好幾年了 (t¯a d˘a le qi´u d˘a h˘ao j˘ı ni´an le) → ungrammatical
Reference: Chinese: a linguistic introduction by Chao Fen Sun