Remove the sear bar assembly and check that the channel is clean and the parts free of foreign matter. Check the sear to see if it has been filed down or damaged. Remove the firing pin and see if someone may have filed it in an effort to lighten the trigger pull. Also check the disconnector pin for free movement and the side plate L-arm and the trigger to make sure they haven't been fooled with.
If you have another Luger, try swapping parts one at a time to see if you can identify the bad part or parts.
What you describe sounds like the sear is engaging the firing pin enough to retract it but releasing it as the action goes into battery. However, if that were the case, it would likely continue firing each time the action worked.
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