According to normative Portuguese grammar (not allowing for exceptions), these are the correct "sequences of tenses":
Duvido que ele tenha ajudado (I doubt he has helped.)
Duvidei que ele ajudasse (I doubted he would help.)
Duvidei que ele tivesse ajudado (I doubted he had helped.)
Eu duvido que ele tenha ajudado. (I doubt he has helped.)
Unless DL's sentence is an exception, the imperfect subjunctive after a present-tense verb in the main clause is ungrammatical. The present perfect subjunctive is needed here.