Could be a weak disconnector spring (sear bar plunger spring) or over rotation of the trigger bar (angled piece inside the side plate). Check with Tom Heller (Lugerdoc) at email
hellerarms@webtv.net He has spare parts and much know how to fix this problem. It is fairly common on Luger I restore. Also Wolff springs sells the trigger return spring if yours is weak. Tom also probably has those if yours is weaken or missing.