According Imam Malik, (only) is it correct not to double the Iqamah when the call to salaat is call. From my understanding when a person shave his beard he committed a sin and if we pray behind that person we take on a sin, is it true or false. Therefore, should the second question be true then how long after one stop shaving before someone can safely pray behind that person without bring harm to himself.
thus, we should not encourage this sin by praying behind him. That’s why we should try not to pray behind someone whose beard is trimmed or shaved. If there is no excuse in praying behind this person (for example, we ourselves have kept this person as the Imam all of our masjid, or made him the Imam of our Jamaat etc.), then by praying behind him we are in actuality encouraging him to continue committing this grave sin. If this person does taubah (repents) and stops shaving or trimming his beard then it is permissible to start praying behind him; there is no time limit in waiting for the beard to grow.